The Flexible Alternating Current Transmission systems(FACTs) devices is power electronics constructed system used to increase the capability of the transmission line. It has several types with different functions for example: SVC, STATCOM,DSTATCOM, TCSC, SSSC, IPFC and UPFC. The Static Synchronous Series Compensator (SSSC) used to control the active power of the transmission line by injecting controllable ac voltage in the series with the transmission line. The Static Synchronous Compensator (STATCOM) employed to control the reactive power by injecting adjustable ac current in the transmission line. The Unified Power Flow Controller (UPFC) consists of SSSC and STATCOM linked by DC capacitor, it controls both active and reactive power flow of the line. This paper presents a simulation study to compare between SSSC, STATCOM and UPFC by inserting each device separately on a 100 MVA, 500 KV, four busses power system. Each converter is designed by 48-pulse multilevel inverter to reduce harmonics contents of the line voltage. This study has been done using MATLAB / SIMULINK package.