This thesis is dedicated to the impact of the Unified Power Flow Controller on PV curves as a criterion for determining the voltage stability. In this work we describe the phenomena of voltage instability in electric power system, the definition of instability phenomena, its formation and also the devices for voltage instability reduction. We specifically describe double busbar system including Unified Power Flow Controller (UPFC) for regulation power flow. There are also presented some of the mathematical bases of UPFC device for adjusting voltage instability problems. In the last chapter this bases result in PV curves with various parameters of the Unified Power Flow Controller.