This paper proposes an obstacle avoidance method for an autonomous mobile robot by using a single ultrasonic sensor. Behaviour based control architecture is implemented rather than the hierarchical method, as it is more flexible and robust. Deriving from the subsumption architecture, new behaviour based hybrid architecture is introduced in this paper. The process of behaviour based planning uses the concept of fixed priority arbitration to coordinate the behaviours. The mobile robot has 5 layers of control architecture including wandering, obstacle avoidance, goal seeking, deadlock recovering and emergency avoidance. Some realistic scenarios are used to test the versatility and robustness of the robot. The results show that the architecture is fast, simple and effective in its mission.