diff --git a/main.go b/main.go
index 4f91e21..60fc6ed 100644
--- a/main.go
+++ b/main.go
@@ -133,6 +133,7 @@ func main() {
 		cli.IntFlag{"q", 0, "QoS", ""},
 		cli.StringFlag{"cafile", "", "CA certificates", ""},
 		cli.StringFlag{"cert", "", "Client certificates", ""},
+		cli.StringFlag{"key", "", "Client private key", ""},
 		cli.StringFlag{"i", "", "ClientiId. Defaults random.", ""},
 		cli.StringFlag{"m", "test message", "Message body", ""},
 		cli.BoolFlag{"r", "message should be retained.", ""},
diff --git a/mqtt.go b/mqtt.go
index 772081b..8b9b436 100644
--- a/mqtt.go
+++ b/mqtt.go
@@ -137,6 +137,18 @@ func NewOption(c *cli.Context) (*MQTT.ClientOptions, error) {
 		TLSConfig.ClientAuth = tls.RequireAndVerifyClientCert
 		TLSConfig.ClientCAs = certPool
 	}
+	key := c.String("key")
+	if key != "" {
+		scheme = "ssl"
+		if cert == "" {
+			return nil, fmt.Errorf("key specified but cert is not specified")
+		}
+		cert, err := tls.LoadX509KeyPair(cert, key)
+		if err != nil {
+			return nil, err
+		}
+		TLSConfig.Certificates = []tls.Certificate{cert}
+	}
 	insecure := c.Bool("insecure")
 	if insecure {
 		TLSConfig.InsecureSkipVerify = true