Abstract: Sub-synchronous oscillation (SSO) is a specific dynamic behavior due to the coupling of mechanic and electrical parts of power system, which is a severe stability problem. Unified Power Flow Controller (UPFC), as the most advanced FACTS device, can restrain the system oscillation effectively through proper design. In this paper, UPFC’s ability to damp SSO was studied first. Then a supplementary sub-synchronous oscillation controller of unified power flow controller based on modal control was proposed. Damping analysis and simulation indicates that this designed UPFC damping controller is capable of SSO mitigation, which can guarantee a safe and stable operation of the system.