Through high voltage lines generating units are connected to load centers to form a power system. The generated voltage steppes up before entering a transmission line, which moves large amount of power over a long distance to a distribution system. Nowadays, power systems in Ethiopia with vast, interconnected networks exchange generation reserves to boost power system's reliability. While, transmitting power in high voltage lines over a long distance, transmission line losses are substantial, which led to abnormal operation of the power system. As a result; electrical loads are facing voltage fluctuation. Due to this issue, transmission line power flow has been regulated by using Flexible Alternating Current Transmission System (FACTS) device such as Unified Power Flow Controller (UPFC). To study the potential of minimization of power loss and improvement of voltage levels in a transmission line, Fuzzy Logic Controller (FLC) based UPFC is proposed for this thesis work. Because, FLCs are easily adaptable to dynamic change in power system operation and is employed for control of UPFC's series and shunt converters. The functionality of this controller is validated for a 230 kV, 341 km long from Bahir Dar to Alamata transmission line. Newton Raphson load flow analysis method is used for identifying the weakest bus of a transmission line for UPFC placement. Therefore; to demonstrate the performance of FLC based UPFC, over PI based UPFC and without UPFC the same load have been applied. For instance, when using PI based UPFC at the weakest bus of a line for the test system, the receiving end voltage profile is improved by 11% whereas, active and reactive power flow are improved by 13.7% and 8.6% respectively. When the proposed FLC based UPFC is used, the receiving end voltage profile is improved by 14.9%. While, the active and reactive power flow are improved by 21.6% and 15.05% respectively. Also, the power loss is 9.9 MW, 6.0 MW and 4.1 MW respectively without UPFC, with PI based UPFC and with the proposed FLC based UPFC. Therefore, it has been seen from the simulation results that, the proposed FLC based UPFC has a better performance compared to conventional PI controller in terms of regulating power flow, improving voltage profile and reducing power loss in a transmission line. Keywords: UPFC, PI Controller, FLC, VSC, Power Flow Control, Voltage Control, Matlab /Simulink