Extending multiple robot motion planning in unknown workspaces is a interesting scope in the Robot motion planning that in this paper is considered. It is used from radiation robots sensors in order to detect their surroundings and positions of the other robots. In this article a new approach considering two objectives is introduced. Finding safest path is as the first objective. The second objective is achieving a trajectory with minimum length. For finding the safest path, it is used from Voronoi Diagram (VD). To provide the safest path, minimizing the distance to the VS is as important criterion. Due to the VD is a geometric location with a distance from all obstacles to the workspace, therefore it is can be a suitable criteria for finding the safest path. For achieving the shortest path as the secondary objective, it is applied Euclidean distance the current position of the robot to goal position. Algorithm that applied for solving this problem is particle swarm optimization (PSO).