Because of growing demand and restrictions in building new lines Transmission systems are becoming stressed in maintaining the stability of the system. In transmission networks, Flexible AC Transmission System (FACTS) is a power electronic based technology to enhance controllability, stability and power transfer capability of ac transmission system. FACTS devices are found to be very effective for stability followed by a disturbance. Static Synchronous Compensator (STATCOM) which is a shunt device of FACTS family is efficient in regulating voltage either by absorbing or by generating reactive power. Compared to other FACTS devices, STATCOM can provide fast and efficient reactive power support to maintain power system voltage stability. This paper proposes a PI control model that controls the voltage during a disturbance. The proposed controller is implemented under MATLAB/SIMULNK environment. In the simulation test, the PI control shows consistent excellence under various operating conditions, such as different initial control gains, different load levels, and change of transmission network, consecutive disturbances, and a severe disturbance.