A small size self-contained autonomous mobile robot has been implemented as a platform for experimental research of sensor based behavior. This robot can move in two dimensional environment using two driving wheels and have optical and ultrasonic range sensors. Each hardware function of the robot is modularized as a single board computer and master module located in center to control the whole system. In the real environment, the autonomous robot must get environment information from sensors of itself, while it is moving, because the environment is not completely known before moving. In the behavior program, it must monitor sensor information and select the suitable motion. Action mode representation is proposed to describe such behavior.