diff --git a/notifications/build.gradle b/notifications/build.gradle
index d431ce21..12464185 100644
--- a/notifications/build.gradle
+++ b/notifications/build.gradle
@@ -166,11 +166,19 @@ dependencies {
             'org.assertj:assertj-core:3.19.0',
             'org.mockito:mockito-junit-jupiter:3.10.0',
             'com.nhaarman.mockitokotlin2:mockito-kotlin:2.2.0',
+            'io.mockk:mockk:1.11.0',
+            'io.mockk:mockk-common:1.11.0',
+            'io.mockk:mockk-dsl:1.11.0',
+            'io.mockk:mockk-dsl-jvm:1.11.0',
+            'io.mockk:mockk-agent-api:1.11.0',
+            'io.mockk:mockk-agent-common:1.11.0',
+            'io.mockk:mockk-agent-jvm:1.11.0',
             "org.junit.jupiter:junit-jupiter-api:${junit_version}"
     )
     testRuntimeOnly("org.junit.jupiter:junit-jupiter-engine:${junit_version}")
     testCompile "org.opensearch.test:framework:${opensearch_version}"
     testCompile "org.jetbrains.kotlin:kotlin-test:${kotlin_version}"
+    testCompile "org.jetbrains.kotlin:kotlin-reflect:${kotlin_version}" // required by mockk
     testCompile "org.mockito:mockito-core:3.10.0"
     testCompile 'com.google.code.gson:gson:2.8.7'
     testImplementation 'org.springframework.integration:spring-integration-mail:5.5.0'
diff --git a/notifications/src/test/kotlin/org/opensearch/notifications/action/SendNotificationActionTests.kt b/notifications/src/test/kotlin/org/opensearch/notifications/action/SendNotificationActionTests.kt
new file mode 100644
index 00000000..007d1601
--- /dev/null
+++ b/notifications/src/test/kotlin/org/opensearch/notifications/action/SendNotificationActionTests.kt
@@ -0,0 +1,99 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ *
+ * Modifications Copyright OpenSearch Contributors. See
+ * GitHub history for details.
+ */
+package org.opensearch.notifications.action
+
+import com.nhaarman.mockitokotlin2.verify
+import io.mockk.every
+import io.mockk.mockkObject
+import org.junit.jupiter.api.BeforeEach
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.extension.ExtendWith
+import org.mockito.Answers
+import org.mockito.ArgumentMatchers.eq
+import org.mockito.Mock
+import org.mockito.Mockito.mock
+import org.mockito.Mockito.times
+import org.mockito.junit.jupiter.MockitoExtension
+import org.opensearch.action.ActionListener
+import org.opensearch.action.support.ActionFilters
+import org.opensearch.client.Client
+import org.opensearch.common.xcontent.NamedXContentRegistry
+import org.opensearch.commons.notifications.action.SendNotificationRequest
+import org.opensearch.commons.notifications.action.SendNotificationResponse
+import org.opensearch.commons.notifications.model.ChannelMessage
+import org.opensearch.commons.notifications.model.EventSource
+import org.opensearch.commons.notifications.model.Feature
+import org.opensearch.commons.notifications.model.SeverityType
+import org.opensearch.notifications.send.SendMessageActionHelper
+import org.opensearch.tasks.Task
+import org.opensearch.transport.TransportService
+
+@ExtendWith(MockitoExtension::class)
+internal class SendNotificationActionTests {
+
+    @Mock
+    private lateinit var transportService: TransportService
+
+    @Mock(answer = Answers.RETURNS_DEEP_STUBS)
+    private lateinit var client: Client
+
+    @Mock
+    private lateinit var xContentRegistry: NamedXContentRegistry
+
+    @Mock
+    private lateinit var listener: ActionListener<SendNotificationResponse>
+
+    private val actionFilters = ActionFilters(setOf())
+
+    private lateinit var sendNotificationAction: SendNotificationAction
+
+    @BeforeEach
+    fun setUp() {
+        sendNotificationAction = SendNotificationAction(
+                transportService, client, actionFilters, xContentRegistry)
+    }
+
+    @Test
+    fun doExecute() {
+        val notificationId = "notification-1"
+        val task = mock(Task::class.java)
+        val request = createSendNotificationRequest()
+        val response = SendNotificationResponse(notificationId)
+
+        // Mock singleton's method by mockk framework
+        mockkObject(SendMessageActionHelper)
+        every { SendMessageActionHelper.executeRequest(request) } returns response
+
+        sendNotificationAction.execute(task, request, listener)
+        verify(listener, times(1)).onResponse(eq(response))
+    }
+
+    private fun createSendNotificationRequest(): SendNotificationRequest {
+        val notificationInfo = EventSource(
+                "title",
+                "reference_id",
+                Feature.REPORTS,
+                SeverityType.HIGH,
+                listOf("tag1", "tag2")
+        )
+        val channelMessage = ChannelMessage(
+                "text_description",
+                "<b>htmlDescription</b>",
+                null
+        )
+        return SendNotificationRequest(
+                notificationInfo,
+                channelMessage,
+                listOf("channelId1", "channelId2"),
+                "sample-thread-context"
+        )
+    }
+}
diff --git a/notifications/src/test/resources/mockito-extensions/org.mockito.plugins.MockMaker b/notifications/src/test/resources/mockito-extensions/org.mockito.plugins.MockMaker
new file mode 100644
index 00000000..ca6ee9ce
--- /dev/null
+++ b/notifications/src/test/resources/mockito-extensions/org.mockito.plugins.MockMaker
@@ -0,0 +1 @@
+mock-maker-inline
\ No newline at end of file