A decentralized control strategy for a robot<br />swarm is presented, where each robot try to form a regular<br />tetrahedron with its three neighbors. The proposed method<br />is based on virtual spring and demands minimum local<br />information. Four neighboring robots can form a regular<br />tetrahedron formation regardless of their initial positions. A<br />neighbor selection procedure is used to extend the strategy<br />In addition, an obstacle avoidance mechanism, based on<br />artificial physics, is also introduced. As a result, the basic<br />swarm behaviors such as aggregation, flocking and obstacle<br />avoidance are demonstrated through simulation