This paper proposes a simple approach to designing an obstacle detection system using ar-drone quadcopter with ultrasonic sensor. The detection system uses some of API to connecting the sensor system and quadcopter system. The result of sensor readings transmitted into quadcopter system through serial communication using node serial-port API. Node ar-drone has a function to convert the sensor reading into a command and would be executed using node.js to make the quadcopter move to avoid the obstacle object. The idea of this research is to make an obstacle detection system using low cost sensor for quadcopter. The result show the system can work successfully to detect and avoid the obstacle object.