Due to rapid increment of power demand, there is massive growth of interconnection which has given rise to numerous challenges in the power network.When a given disturbances are happened at a specific location,it is hard to find due to network complexity.Those disturbances leads synchronous generators to electro-mechanical oscillation,so the power system looses its stability.Among them Voltage instability is one of the phenomena which have resulted in a major blackout in power system .Efficient utilization of this interconnected network is the major priority to satisfy new load demands.Nowadays Power System stabilizers(PSS) and flexible alternating current transmission system(FACTS) devices are emerged for oscillation damping in the power systems.This thesis presents the effectiveness of fuzzy based power system stabilizers(PSS) and unified power flow controller(UPFC) on voltage stability improvement by considering North West Region (NWR) high voltage transmission line with Tana Beles, Fincha and Tekeze hydropower generation plants .In this thesis Fuzzy based PSS is designed to supply a damping signal by minimizing the rotor speed deviation of each synchronous generators and UPFC is implemented between Bahir Dar 400 and Markos 400 bus .The proposed system had been built in MATLAB/Simulink software and comparison of system parameters such as generators terminal voltage, rotor angle, rotor speed, net power and bus voltage variations have been compared also for different disturbances without PSS, with conventional PSS , fuzzy based PSS and also fuzzy PSS with UPFC.When single line to ground fault between Mota and Markos line is applied for 0.01s,the system is unstable with out PSS but with CPSS, FPSS and FPSS with UPFC,the system becomes stable and Mota bus voltage has high peak of 1.042pu, 1.025pu,1.02pu and settling time of 4.2s, 1.5s, 0.5s respectively.When 0.01pu magnitude of step input disturbance is applied at Beles reference voltage for 0.01s ,Mota bus voltage with CPSS, FPSS and FPSS with UPFC has high peak of 1.023pu, 1.022pu,1.02pu and settling time of 5.5s, 2.8s, 2s respectively.From the results we can conclude that, fuzzy based PSS has better voltage stability performance than CPSS in terms of settling time and overshoots for small and large disturbances.Integrating UPFC with FPSS has great importance not only for reducing oscillation on the power system network but also it improves voltage profile during and after disturbance which is the main criteria of voltage stability in the power system. Keywords: FACTS devices; Fuzzy logic controllers (FLC); Power system stabilizer (PSS); Unified Power Flow Controller (UPFC); voltage stability