Currently, in Wireless Sensor Networks (WSNs), the main idea in most localization algorithms has been that a mobile anchor node, e.g., GPS-equipped (Global Positioning System) nodes, broadcasts its coordinates to locate unknown nodes. In this case, a basic problem is the path planning of the mobile anchor node which should move along the trajectory to minimize the localization error and locate the unknown nodes. In this paper, we propose a Localization algorithm with a Mobile Anchor node based on Trilateration (LMAT). LMAT algorithm uses a mobile anchor node to move according to the equilateral triangle trajectory in deployment area. Simulation results show that the performance of our LMAT algorithm is better than that of other similar algorithms, e.g., SPIRAL, SCAN, DOUBLE SCAN and HILBERT algorithms.