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Unified power flow controller (UPFC) is one of the most versatile and complex flexible AC transmission system devices to have emerged with a proven capability of instantaneous control of transmission line parameters. This study presents an approach based on the direct Lyapunov stability theory with finite-time convergence and chattering free characteristics to improve the power flow control in transmission lines using a UPFC. A state variable control strategy is derived and implemented to tackle the problems of reference tracking, robustness against parameter uncertainty and external disturbances. The main goal of the presented control system is power flow control with finite-time convergence of system states'. The chattering phenomena and discontinuity of the controller that is common in finite-time controllers are also removed to obtain a continuous and smooth controller. Simulation results are given to illustrate the effectiveness of the proposed algorithm. It is shown that the settling time of the system enhanced with the proposed controller is significantly less than the conventional non-linear controllers. As the most simply measurable states of the system are used in the suggested controller, there is no need to design a state space variable observer system. The proposed controller is investigated on the UPFC connected to a twobus power system.