Flexible AC Transmission Systems (FACTS) devices play a crucial role in modern power systems by providing advanced control capabilities and improving the overall performance, reliability, and efficiency of electrical grids. In an electrical power transmission network, a Unified Power Flow Controller (UPFC) is a device that provides fast control of active and reactive power flow in the lines. It consists of a set of back-to-back voltage source converters based on GTO coupled in series and shunt configurations over a common DC voltage link. Usually, UPFC is employed in a long-distance transmission system. Further occurrence of fault in transmission line due to Lightning phenomenon is very common. So, there is a need of fault detection scheme to ensure the safe and reliable operation of the UPFC transmission system. In this paper, fuzzy inference system (FIS) is used for fault detection of unsymmetrical faults in UPFC transmission system.