A simple and novel motion planning method for mobile robot is presented in this paper. Motion planning is achieved by adjusting the robot behavior according to information from sensors based on the relative coordinates. The robot has two behaviors in the course of planning:moving to the goal and collision avoidance, and the collision avoidance behavior has the higher priority. The switch of two behaviors is based on the acceleration space coordinates. The problem of collision avoidance is solved, and the behavior of moving to the goal is considered as a reverse problem of the avoidance. Simulation results verify the validity.