Robot deployment in unknown region is one of the most important issues in wireless sensor networks (WSNs). Based on the Obstacle-Free Robot Deployment Mechanism (ORRD), we propose a new deployment mechanism. This mechanism attempts to use the least number of sensors to guarantee full coverage and avoid physical obstacles in unknown region. We extend the mechanism to support multiple robots that move independently and asynchronously. When robots meet each other, they explore the way to efficiently and cooperatively deploy sensors for full coverage and obstacle resistance. The simulation results show that the proposed mechanism outperforms the existing deployment mechanism in terms of coverage ratio, deployment efficiency, and the communication overhead.