This paper presents a 3D path planning algorithm for an unmanned aerial vehicle(UAV)in complex environments.In this algorithm,the environments are divided into voxels by octree algorithm.In order to satisfy the safety requirement of the UAV,free space is represented by free voxels,which have enough space margin for the UAV to pass through.A bounding box array is created in the whole 3D space to evaluate the free voxel connectivity.The probabilistic roadmap method(PRM)is improved by random sampling in the bounding box array to ensure a more efcient distribution of roadmap nodes in 3D space.According to the connectivity evaluation,the roadmap is used to plan a feasible path by using A*algorithm.Experimental results indicate that the proposed algorithm is valid in complex 3D environments. This paper presents a 3D path planning algorithm for an unmanned aerial vehicle (UAV) in complex environments. This algorithm, the environments are divided into voxels by octree algorithm. In order to satisfy the safety requirement of the UAV, free space is represented by free voxels, which have enough space margin for the UAV to pass through. A bounding box array is created in the whole 3D space to evaluate the free voxel connectivity. The probabilistic roadmap method (PRM) is improved by random sampling in the bounding box array to ensure a more efcient distribution of roadmap nodes in 3D space. According to the connectivity evaluation, the roadmap is used to plan a feasible path by using A * algorithm. Experimental results indicate that the proposed algorithm is valid in complex 3D environments.