A new sensor network architecture called active sensor network (ASN) is proposed in this paper. By integrating multiple small, sensor network-friendly mobile robots into a traditional sensor network, a closed-loop, dynamic, adaptive sensor network is formed. Such sensor networks have the following merits: adaptivity, self healing, responsiveness and longer lifetime. This paper focuses on the distributed sensor node localization using multiple mobile robots. A potential-based robot area partition algorithm and a localization algorithm are developed. Simulation results verify the proposed algorithms.