diff --git a/index_mjs.mjs b/index_mjs.mjs
new file mode 100644
index 0000000..529deb2
--- /dev/null
+++ b/index_mjs.mjs
@@ -0,0 +1,9 @@
+// For development/testing purposes
+export const handler = (event, context, callback) => {
+  console.log('Running index.handler (mjs)')
+  console.log('==================================')
+  console.log('event', event)
+  console.log('==================================')
+  console.log('Stopping index.handler (mjs)')
+  callback(null)
+}
diff --git a/lib/main.js b/lib/main.js
index b9e5108..d23baea 100644
--- a/lib/main.js
+++ b/lib/main.js
@@ -59,7 +59,18 @@ event_sources.json and ${program.eventFile} files as needed.`)
 
     this._createSampleFile(program.eventFile, 'event.json')
     const splitHandler = program.handler.split('.')
-    const filename = splitHandler[0] + '.js'
+    const filename = (() => {
+      for (const extension of ['.js', '.mjs']) {
+        if (fs.existsSync(splitHandler[0] + extension)) {
+          return splitHandler[0] + extension
+        }
+      }
+    })()
+    if (filename == null) {
+      console.error('Handler file not found.')
+      process.exitCode = 255
+      return
+    }
     const handlername = splitHandler[1]
 
     // Set custom environment variables if program.configFile is defined
diff --git a/test/node-lambda.js b/test/node-lambda.js
index d4d5481..33c216d 100644
--- a/test/node-lambda.js
+++ b/test/node-lambda.js
@@ -295,6 +295,54 @@ describe('bin/node-lambda', () => {
         })
       })
     })
+
+    describe('node-lambda run (by *.mjs)', () => {
+      it('`node-lambda run` by index.mjs', function (done) {
+        const run = spawn('node', [
+          nodeLambdaPath, 'run',
+          '--handler', 'index_mjs.handler'
+        ])
+        let stdoutString = ''
+        run.stdout.on('data', (data) => {
+          stdoutString += data.toString().replace(/\r|\n|\s/g, '')
+        })
+
+        run.on('exit', (code) => {
+          const expected = 'Runningindex.handler(mjs)' +
+            '==================================' +
+            'event{asyncTest:false,callbackWaitsForEmptyEventLoop:true,callbackCode:\'callback(null);\'}' +
+            '==================================' +
+            'Stoppingindex.handler(mjs)Success:'
+          assert.equal(stdoutString, expected)
+          assert.equal(code, 0)
+          done()
+        })
+      })
+    })
+
+    describe('node-lambda run (handler file not found)', () => {
+      it('`node-lambda run` Invalid handler specification.', function (done) {
+        const run = spawn('node', [
+          nodeLambdaPath, 'run',
+          '--handler', 'not_found.handler'
+        ])
+        let stdoutString = ''
+        run.stdout.on('data', (data) => {
+          stdoutString += data.toString().replace(/\r|\n|\s/g, '')
+        })
+        let stderrString = ''
+        run.stderr.on('data', (data) => {
+          stderrString += data.toString()
+        })
+
+        run.on('exit', (code) => {
+          assert.equal(stdoutString, '')
+          assert.match(stderrString, /Handler file not found\./)
+          assert.equal(code, 255)
+          done()
+        })
+      })
+    })
   })
 
   describe('node-lambda duplicate check of short option', () => {