Abstract: The Unified Power Flow Controller (UPFC) is the one of the FACTS devices which can control power system parameters such as terminal voltage, line impedance and phase angle. The UPFC is not only power flow controller but also power system stabilizing controller. The UPFC is a combination of Static synchronous Compensator (STATCOM) and a Static Synchronous Series Compensator (SSSC) which are coupled via a common dc link to allow bi-directional flow of real power between the series output terminals of the SSSC and the shunt output terminals of the STATCOM. UPFC is the one FACTS device which can control real and reactive power flow. UPFC can control the power system parameters such as terminal voltage, line impedance and phase angle. UPFC control provides additional damping during oscillations. The dynamic models of UPFC are used to design suitable controllers for power flow, voltage and damping controls. Fuzzy logic controller based UPFC is taken in this research. The parameters of UPFC (m_B, m_E, δ_B and δ_e) can be modulated for achieving desired damping in the system. The performance of the four UPFC based damping controllers (m_B, m_E, δ_B and δ_E ) and fuzzy based controllers are investigated under wide variations of the loading conditions in this research. A Single-Machine-Infinite-Bus (SMIB) system installed with UPFC is considered. The static excitation system model type IEEE-ST1A has been considered in this study. The UPFC considered here is assumed to be based on Pulse Width Modulation (PWM) converters.