A flexible ac transmission system (FACTS) controller can play an important role in mitigating voltage stability problems and power transfer capability. The unified power flow controller (UPFC) is one of the FACTS controller that combines both series and shunt compensators, and offers more specific characteristics compared to other devices controller. In this paper, a UPFC is applied to improve dynamic performance of electrical power system by using power flow control. Phasor model of UPFC is used. All the simulations for this work have been carried out using MATLAB/SIMULINK environment.