This paper describes a novel application of genetic algorithm for navigation of an autonomous mobile robot (AMR) under unknown environments. In the navigation system, the AMR is controlled by the decision- making block, which consists of neural network. To achieve both successful navigation to the goal and the suitable obstacle avoidance, the connection weights of the neural network and speed gains for predefined actions are encoded as genotypes and are tuned simultaneously by genetic algorithm so that the static and dynamic danger-degrees, the energy consumption and the distance and direction errors decrease during the navigation. Experimental results demonstrate the validity of the proposed navigation system.