Abstract: Due to presence of Flexible Alternative Current Transmission System (FACTS) devices such as Unified Power Flow Controller (UPFC) on transmission line, the measured impedance by the distance relay become lower or higher than the actual line impedance. In this study the influence value of UPFC and System conditions parameters on distance protection is investigated. First the comprehensive equation of impedance seen by the relay in presence of UPFC are derived and then, the accurate percentages of over-reaching or under-reaching corresponding to the variation of the UPFC parameters (voltage magnitude, phase angle) and locations are calculated. The studies also include the different power angles of networks and two types of faults. The analytical results are confirmed by the simulation in PSCAD/EMTDC software environment. For correct performance of distance protection a relaying adaptive scheme is required to cope with the problems of over/under-reaching.