The voltage magnitude and phase Angle control of static phase shifter (VMPA–SPS) to damp the subsynchronous resonance(SSR) oscillations is investigated. A linear mathematical model of seriescompensated transmission line power system with static phase shifter is developed .The input control signals to the simulated power system is the phase angle and voltage magnitude deviations of static phase shifter. A controllability measure based on singular value decomposition (SVD) is used to identify the effectiveness of each control input signal on the electromechanical modes. A state feedback supplementary controller based on the linear quadratic regulator principle with a full rank observer is used to modulate the voltage magnitude and phase angle deviations of SPS to stabilize the SSR modes under different operating conditions and compensation levels of the compensated transmission line. To validate the effectiveness of the proposed supplementary controller , the studied power system is subjected to different disturbances. The digital simulation results prove the powerful of the proposed static phase shifter supplementary controller in terms of the fast damping of the SSR oscillations with less overshoot/undershoot.