Flexible AC Transmission System (FACTS) controllers can be divided into three categories, series controller, shunt controller, and combined series-shunt controller. Among these FACTS controllers, Unified Power Flow Controller (UPFC) is a representative combined series-shunt controller. UPFC is conceptually composed of the Static Synchronous Series Compensator (SSSC) and the Static Compensator (STATCOM). Difficulties in modeling UPFC in the conventional dynamic simulation program arise from the fact that the injected voltage by series inverter (SSSC) and the shunt inverter (STATCOM) side voltage are dependent on each other. Therefore, the relationship is implicit and nonlinear. This paper presents a Newton-type current injection model of the UPFC for overcoming these difficulties. This method can also be applied to another FACTS model such as SSSC and STATCOM and to the power system with multiple FACTS devices. The case study results in this paper show that the proposed model is efficient for studying the effects of the UPFC and can be applied to another FACTS models such as SSSC and STATCOM on the power system dynamics.