Abstract : Efficient path planning is an essential issue for USV navigation. In order to find a safe route with the optimal distance cost, the A* algorithm is one of the effective algorithms.  In this paper, the modified A* algorithm is applied for path planning of an unmanned surface vehicle in static and dynamic obstacle environment. The current study adopts an A* approach with a safety distance between USV and obstacles. Two important parameters such as path length and computational time are considered with the different start time. The results show that the modified approach is effective for obstacle avoidance of an unmanned surface vehicle which is compliant with the International Regulations for Avoiding Collision at sea (COLREGs).