Abstract: The node localization technology in wireless sensor networks was studied, and a flexible trilateration localization (FTL) based on RSSI was presented. In the FTL process three cooperant beacons are taken to generate sensors' effective error ranges where the unknown nodes exist in, thus completing the node localization with the assistance by one or two hops of neighboring nodes. Simulation results showed that FTL needs low request for the positions of sensors relative to beacons in comparison to the RSSI-based distributed node localization using three common beacons. When the mean ranging error is 10%, the mean localization error is about 20% of sensors' RF communication distance.