This paper presents a simple method of evaluating the first swing stability of a large power system in the presence of Unified Power Flow Controller (UPFC) device. First a UPFC and the associated transmission line are considered and represented by an equivalent p—circuit model. The above model is then interfaced to the power network to obtain the system reduced admittance matrix which is needed to generate the machine swing curves. The above p —circuit model can also be used to represent other FACTS devices (SSSC and STATCOM) by selecting appropriate values of control parameters of the UPFC. The complex voltage at two end buses of the p-circuit model is also evaluated during simulation to implement various existing control strategies of FACTS devices and to update the reduced admittance matrix. The effectiveness of the proposed method of generating dynamic response and hence evaluating first swing stability of a power system in the presence of UPFC device will be tested on the ten-machine New England system.