Abstract: The focus of this research work is on a FACTS device known as Unified Power Flow Controller (UPFC), which can provide simultaneous control of basic power system parameters like voltage, active power flow, reactive power flow, impedance and phase angle. In this research work, simulation models for different multi machine systems are carried out, e.g. a hybrid power system, a power system with two synchronized hydro power stations etc., without UPFC & with UPFC located at the load end of the power system, has been developed. Simulation models have been incorporated into MATLAB based Power System Toolbox (PST) for their voltage stability analysis.