Abstract: The current two-dimension path planning technologies are adequate for semi-autonomous Unmanned Aerial Vehicles (UAV) that operate in a relatively structured environment. However, for UAVs in a complicated uncertain environment the present techniques are inadequate. A three-dimensional (3D) path planning algorithm of UAV which based on improved A* algorithm is proposed in this paper in order to solve problem of path planning for UAV in complex environment. First, we attempt to extend the search space of traditional A* algorithm to three-dimension. Then we make the heuristic algorithm suitable for UAV with various constraints. Finally, in simulation experiments, the effectiveness of improved A* algorithm is verified concerning the accurateness, safety and the adaptability.