Collision avoidance is one of the core problems in designing a multi-unmanned aerial vehicle (UAV) system. It is to ensure each UAV can reach its target without colliding with any moving or static obstacles. With increasing usage of UAVs in many application areas, research on collision avoidance algorithm has become a hot topic. Several approaches on collision avoidance have been reported. In this paper, we design a three-dimensional collision avoidance on multi-UAV systems based on the original three-dimensional velocity obstacle method.