In wireless sensor networks, relative localization problem is to infer the relative locations instead of absolute locations. This paper considers one dimensional relative localization problem in wireless sensor network, and proposes PLO (Proximity List algOrithm), which characterizes each node by its Proximity List, a list of all node IDs ordered by distances away. A trace-driven analysis verifies that the difference in distances can be mostly reflected by the difference in received RSSI (received signal strength indicator) values. Therefore, proximity lists can be obtained by comparing RSSI values. Finally, relative locations are obtained by locating end nodes in the line topology. This paper shows that this algorithm is feasible in practical situations.