A power system is a complex, nonlinear, and dynamic system, with its operating parameters continuously changing with time. High-voltage transmission power systems are frequently subject to transient instability problems due to the complexity of large load lines and faults, which results in power losses and higher voltage deviation. This problem can lead to catastrophic events such as cascading failure and/or widespread blackouts. Based on the collected data in 2021 G.C from the EEP office, Ethiopia's high-voltage transmission power system in the North-West region is a victim of this problem. Due to the growth in demand for electrical energy, the maximum capacity of the transmission lines should be increased to ensure a secure and uninterrupted power supply to consumers. This can be achieved by means of the installation of flexible AC transmission system (FACTs) devices. To address the problem, this thesis uses ANFIS-based UPFC for high-voltage transmission lines. Preference was given to this device for its fast-acting reactive power compensation on high-voltage transmission networks. The PSO algorithm was used to locate the optimal placement of the UPFC. Voltage error and rate of change of voltage error, which are extracted from PI-based UPFCs, are the inputs to the ANFIS controller. Of the extracted data, 70% is used for ANFIS training, and 30% is used for ANFIS testing. Different disturbances, such as three-phase with ground fault and single-phase with ground fault, are used to demonstrate the performance of the suggested controller. For instance, when a three-phase with ground fault occurs at the midpoint of the Beles to Bahir Dar transmission line, the settling time of rotor angle deviation, rotor speed, rotor speed deviation, and output active power of synchronous generators using ANFIS-based UPFC is reduced by 70.58 %, 37.75 %, 37.75 %, and 43.75 %, respectively, compared to a system without UPFC. The active power loss reduced by PI-based UPFC and ANFIS-based UPFC on the Beles to Bahir Dar transmission line at peak load is 51.25 % and 71.50 %, respectively, compared to a system without UPFC. On the other hand, the active power loss reduced by PI-based UPFC and ANFIS-based UPFC on the Bahir Dar to D/Markos transmission line at peak load is 20 % and 47.9 %, respectively, compared to a system without UPFC. ANFIS-based UPFC provides a better result than PI-based UPFC for transient stability enhancement in terms of percentage overshoot and settling time. Keywords: FACTs, UPFC, PI Controller, ANFIS, power system, PSO