The aim of this book is to investigate and improve the transmission line loadability, various FACTS devices have been considered in detail and a comparative analysis is made to select the optimal one. The selected controller is further investigated in detail in controlling the power flow over the transmission line to improve the transmission line loadability. Voltage injection of variable series converter phase angle model is used to study the performance of the Controller in controlling the active and reactive power flow in the line. The model is also incorporated in the Newton Raphson algorithm for load flow study. Simultaneous method is used in formulating the equations of the UPFC and the power flow balance equations of the line and is combined in to one set of non – linear algebraic equations. The Unified Power Flow controller (UPFC) is placed at nine different possible locations within the transmission line to choose the optimal placement. Simulation is done using MATLAB-R2007a Program. The result of the analysis without and with UPFC is compared in terms of active and reactive power flow in the line and relevant recommendation has been made.