Cannot communicate with Serial Monitor - Arduino Yun

I connect Arduino Yun to my Computer with WiFi. I was able to get the IP address of my Arduino Yun. However, when I try to print something. It seems to not work at all. The following code is the one that I try:

void setup() {
 Serial.begin(9600);       
}

void loop() {  
 // print labels 
 Serial.print("NO FORMAT");  
 delay(100);
}

I did not get any kind of error. However, when I open the Serial Monitor, it shows nothing at all. I try to use the same code with Arduino Uno. It works fine.