The objective of this research is to analyze the application of FACTS devices to increase the maximum load ability of the transmission lines which may be constrained by a transient stability limit. The maximum capacity of transmission lines needs to be raised in response to the increasing demand for electricity in order to guarantee a safe, continuous power supply to consumers and to prevent system voltage disturbances other than those caused by transmission line load variation. Whenever a Power system is subjected to sudden changes or disturbance in load levels then stability is the main concept that determines the stable operation of the power system. This can be achieved by means of the installation of flexible AC transmission system (FACTs) devices. To address this problem, this research uses Fuzzy based UPFC on Bahir Dar to Debre Markos high voltage transmission line. Fuzzy logic controllers (FLCs) can handle non-linearity and uncertainties in a system more effectively than PI controllers. FLCs can capture complex relationships between inputs and outputs without requiring a precise mathematical model of the system. On the other hand, PI controllers are simpler and more straightforward to implement, but it requires more tuning to perform well in systems with nonlinearities or uncertainties. Voltage error and rate of change of voltage error, which are extracted from Fuzzy based UPFCs, are the inputs to the Fuzzy logic controller. A literature was given guidance for the fast tuning of UPFC perimeters and proper placement of UPFC on the transmission line. 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. When a three-phase with ground fault occurs at the sending end line of Bahir Dar to Debre Markos transmission line, the settling time of rotor angle deviation(Δδ), active power flow, rotor speed deviation(Δω) and rotor speed(ωm) using Fuzzy-based UPFC reduced by 12.5%,10.76%,14.96% and 9.1 percent, respectively, and when single-phase to ground fault occurred on Bahir Dar to Debre Markos transmission line, Fuzzy-based UPFC reduced the settling time of rotor angle deviation(Δδ), active power flow, rotor speed deviation(Δω) and rotor speed(ωm) by 10.05%,11.5%,13.4% and 7.62 percent, respectively, as compared to a system without UPFC. Therefore, using fuzzy based UPFC improved transient stability limit for power transmission line by MATLAB software. Keywords: FACT device, Transient stability, Transmission line, UPFC