diff --git a/images/virtualization-artifact/pkg/controller/vm/internal/defaulter/core_fraction_test.go b/images/virtualization-artifact/pkg/controller/vm/internal/defaulter/core_fraction_test.go
new file mode 100644
index 0000000000..227fb2b05d
--- /dev/null
+++ b/images/virtualization-artifact/pkg/controller/vm/internal/defaulter/core_fraction_test.go
@@ -0,0 +1,246 @@
+/*
+Copyright 2025 Flant JSC
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+     http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package defaulter_test
+
+import (
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"sigs.k8s.io/controller-runtime/pkg/client"
+
+	"github.com/deckhouse/virtualization-controller/pkg/common/testutil"
+	"github.com/deckhouse/virtualization-controller/pkg/controller/vm/internal/defaulter"
+	"github.com/deckhouse/virtualization/api/core/v1alpha2"
+	"github.com/deckhouse/virtualization/api/core/v1alpha3"
+)
+
+var _ = Describe("CoreFractionDefaulter", func() {
+	var (
+		ctx                = testutil.ContextBackgroundWithNoOpLogger()
+		coreDefaulter      *defaulter.CoreFractionDefaulter
+		setupCoreDefaulter func(objs ...client.Object)
+		newVM              func(className string, cores int, coreFraction string) *v1alpha2.VirtualMachine
+		newVMClass         func(sizingPolicies []v1alpha3.SizingPolicy) *v1alpha3.VirtualMachineClass
+	)
+
+	BeforeEach(func() {
+		setupCoreDefaulter = func(objs ...client.Object) {
+			GinkgoHelper()
+			fakeClient, err := testutil.NewFakeClientWithObjects(objs...)
+			Expect(err).NotTo(HaveOccurred())
+			coreDefaulter = defaulter.NewCoreFractionDefaulter(fakeClient)
+		}
+
+		newVM = func(className string, cores int, coreFraction string) *v1alpha2.VirtualMachine {
+			return &v1alpha2.VirtualMachine{
+				TypeMeta: metav1.TypeMeta{
+					Kind:       v1alpha2.VirtualMachineKind,
+					APIVersion: v1alpha2.SchemeGroupVersion.String(),
+				},
+				ObjectMeta: metav1.ObjectMeta{
+					Name: "vm",
+				},
+				Spec: v1alpha2.VirtualMachineSpec{
+					VirtualMachineClassName: className,
+					CPU: v1alpha2.CPUSpec{
+						Cores:        cores,
+						CoreFraction: coreFraction,
+					},
+				},
+			}
+		}
+
+		newVMClass = func(sizingPolicies []v1alpha3.SizingPolicy) *v1alpha3.VirtualMachineClass {
+			return &v1alpha3.VirtualMachineClass{
+				TypeMeta: metav1.TypeMeta{
+					Kind:       v1alpha3.VirtualMachineClassKind,
+					APIVersion: v1alpha3.SchemeGroupVersion.String(),
+				},
+				ObjectMeta: metav1.ObjectMeta{
+					Name: "vmc",
+				},
+				Spec: v1alpha3.VirtualMachineClassSpec{
+					SizingPolicies: sizingPolicies,
+				},
+			}
+		}
+	})
+
+	AfterEach(func() {
+		coreDefaulter = nil
+	})
+
+	Context("when coreFraction is already set", func() {
+		It("should not change coreFraction and should not require VMClass", func() {
+			setupCoreDefaulter()
+
+			vm := newVM("any-class", 2, "25%")
+
+			err := coreDefaulter.Default(ctx, vm)
+			Expect(err).NotTo(HaveOccurred())
+			Expect(vm.Spec.CPU.CoreFraction).To(Equal("25%"))
+		})
+	})
+
+	Context("when virtualMachineClassName is empty", func() {
+		It("should keep coreFraction empty", func() {
+			setupCoreDefaulter()
+
+			vm := newVM("", 2, "")
+
+			err := coreDefaulter.Default(ctx, vm)
+			Expect(err).NotTo(HaveOccurred())
+			Expect(vm.Spec.CPU.CoreFraction).To(BeEmpty())
+		})
+	})
+
+	Context("when VMClass cannot be found", func() {
+		It("should return an error", func() {
+			setupCoreDefaulter()
+
+			vm := newVM("non-existing-class", 2, "")
+
+			err := coreDefaulter.Default(ctx, vm)
+			Expect(err).To(SatisfyAll(
+				HaveOccurred(),
+				MatchError(ContainSubstring("failed to get")),
+				MatchError(ContainSubstring("not found")),
+			))
+		})
+	})
+
+	Context("when VMClass has sizing policies", func() {
+		It("should set coreFraction from matching sizing policy defaultCoreFraction", func() {
+			defaultCF := v1alpha3.CoreFractionValue("50%")
+			vmClass := newVMClass([]v1alpha3.SizingPolicy{
+				{
+					Cores: &v1alpha3.SizingPolicyCores{
+						Min: 1,
+						Max: 4,
+					},
+					DefaultCoreFraction: &defaultCF,
+				},
+			})
+
+			setupCoreDefaulter(vmClass)
+
+			vm := newVM(vmClass.Name, 2, "")
+
+			err := coreDefaulter.Default(ctx, vm)
+			Expect(err).NotTo(HaveOccurred())
+			Expect(vm.Spec.CPU.CoreFraction).To(Equal("50%"))
+		})
+
+		It("should set default 100% when matching policy has no defaultCoreFraction", func() {
+			vmClass := newVMClass([]v1alpha3.SizingPolicy{
+				{
+					Cores: &v1alpha3.SizingPolicyCores{
+						Min: 5,
+						Max: 8,
+					},
+				},
+			})
+
+			setupCoreDefaulter(vmClass)
+
+			vm := newVM(vmClass.Name, 6, "")
+
+			err := coreDefaulter.Default(ctx, vm)
+			Expect(err).NotTo(HaveOccurred())
+			Expect(vm.Spec.CPU.CoreFraction).To(Equal("100%"))
+		})
+
+		It("should return error when no policy matches VM cores", func() {
+			defaultCF := v1alpha3.CoreFractionValue("50%")
+			vmClass := newVMClass([]v1alpha3.SizingPolicy{
+				{
+					Cores: &v1alpha3.SizingPolicyCores{
+						Min: 5,
+						Max: 8,
+					},
+					DefaultCoreFraction: &defaultCF,
+				},
+			})
+
+			setupCoreDefaulter(vmClass)
+
+			vm := newVM(vmClass.Name, 2, "")
+
+			err := coreDefaulter.Default(ctx, vm)
+			Expect(err).To(SatisfyAll(
+				HaveOccurred(),
+				MatchError(ContainSubstring("not among the sizing policies")),
+			))
+		})
+
+		It("should set default 100% when coreFractions includes 100%", func() {
+			vmClass := newVMClass([]v1alpha3.SizingPolicy{
+				{
+					Cores: &v1alpha3.SizingPolicyCores{
+						Min: 1,
+						Max: 4,
+					},
+					CoreFractions: []v1alpha3.CoreFractionValue{"50%", "100%"},
+				},
+			})
+
+			setupCoreDefaulter(vmClass)
+
+			vm := newVM(vmClass.Name, 2, "")
+
+			err := coreDefaulter.Default(ctx, vm)
+			Expect(err).NotTo(HaveOccurred())
+			Expect(vm.Spec.CPU.CoreFraction).To(Equal("100%"))
+		})
+
+		It("should return error when coreFractions doesn't include 100% and no defaultCoreFraction", func() {
+			vmClass := newVMClass([]v1alpha3.SizingPolicy{
+				{
+					Cores: &v1alpha3.SizingPolicyCores{
+						Min: 1,
+						Max: 4,
+					},
+					CoreFractions: []v1alpha3.CoreFractionValue{"25%", "50%"},
+				},
+			})
+
+			setupCoreDefaulter(vmClass)
+
+			vm := newVM(vmClass.Name, 2, "")
+
+			err := coreDefaulter.Default(ctx, vm)
+			Expect(err).To(SatisfyAll(
+				HaveOccurred(),
+				MatchError(ContainSubstring("default value for core fraction is not defined")),
+			))
+		})
+	})
+
+	Context("when VMClass has no sizing policies", func() {
+		It("should set default 100%", func() {
+			vmClass := newVMClass(nil)
+
+			setupCoreDefaulter(vmClass)
+
+			vm := newVM(vmClass.Name, 2, "")
+
+			err := coreDefaulter.Default(ctx, vm)
+			Expect(err).NotTo(HaveOccurred())
+			Expect(vm.Spec.CPU.CoreFraction).Should(Equal("100%"))
+		})
+	})
+})