A number of factors, including the slow rate of transmission line development, environmental restrictions, the unregulated nature of the electrical market, etc., have led to the majority of power systems working at or near their operative limitations in recent years. Numerous issues in the power system could be caused by voltage instability Consequently, the biggest concern now is ensuring the stability of the power system. Voltage instability has been identified as the primary cause of recent blackouts worldwide. Between 2017 and 2020, line faults and operational issues caused more than 256 outages on EEP's 400 kV system. Reactive power compensation is the most effective technique to improve the system's capacity to transmit power, lessen the frequency of blackouts, and preserve voltage stability. This study considers both an evaluation of the current voltage profile and a correct solution through modeling of the unified power flow controller (UPFC) in a power system in order to improve the voltage stability of Ethiopia's 400 kV power system. A simulation program called MATLAB is used to compute the voltage stability index. The bus with the highest LINDEX magnitude, reported on the Gelan bus since its magnitude is 0.9303, is determined to be the ideal position for UPFC placement based on the VSI result. (For example, a low voltage magnitude bus that is classified as the weakest bus and has the greatest value of L-INDEX). The computation determines the ideal UPFC size. Employing PSO. Integrating the 175.69MVA, 197.87MVA and 250MVA-rated UPFC within the system improves bus voltage stability of the system significantly with different load scenario(10%,20%and50%) condition respectively. When the load is increased by 10%, 30%, 50% of its base load the voltage profile becomes 0.9285, 0.9215, 0.8738 respectively before UPFC and 0.958, 0.959 and 0.957 after UPFC. By simulating the system using UPFC, the likelihood of a bus voltage magnitude violation is effectively decreased. The results showed that the addition of UPFC to the system greatly enhanced and preserved the power system's voltage stability. KEYWORDS: FACTS device, PSO (Particle swarm optimization), Transmission line capacity, UPFC (Unified power flow controller), VSI (Voltage stability index)