Because of continuous demand in heavily loaded networks, voltage instability problems are common in power systems. FACTS devices are increasingly employed in stabilizing the power systems. Among the FACTS devices, Static Synchronous Compensator (STATCOM) injects the compensating current in phase quadrature with line voltage and can imitate as inductive or capacitive reactance thereby to produce capacitive power for the AC grid or draws inductive power from the AC grid to control power stream in the line. This paper proposes an algorithm named Bat Algorithm, for getting the optimal gain of the PI control of STATCOM under disturbances and helps in improving the performance and attaining a wanted response, irrespective of the variation of working conditions. This work is implemented under MATLAB/SIMULINK environment. And it shows better results over conventional PI and Adaptive PI control. © 2018, Institute of Advanced Scientific Research, Inc. All rights reserved.