The fluctuation of voltage cannot be tolerant for equipment in modern industrial plants such as lighting loads, PLC, robots, and another equipment, which exist in transmission and distribution systems, so we should use proper aids to regulate voltage and control it. In this study a (± 25Mvar) Static Synchronous Compensator (STATCOM) is used to enhance voltage stability in a (66 kv, 1500MV.A) power transmission network. The STATCOM in this study regulates the voltage of the transmission network for changing in voltage (± 7%) from the nominal value. A model of the power transmission system and another model of the STATCOM device, which will enhance the stability of voltage are designed in MATLAB/Simulink. And the control of (STATCOM) is achieved by using a Proportional Integrative (PI) controller with Fuzzy Logic Supervisor to adjust the parameters in PI controller in DC voltage regulator during transient states of load changing which gives more stability in DC voltage. The results of the simulation are shown. This study demonstrates the ability of STATCOM for regulating the voltage of the transmission system by injecting and absorbing reactive power from the power system, and the DC voltage be more stability by using Fuzzy Logic supervisor.