Abstract: The effect of unified power flow controller (UPFC) on the sub-synchronous oscillation (SSO) was analyzed. First, the dq model of UPFC which can be used to study SSO was established. Based on complex torque analysis method, the equivalent impedance of UPFC was obtained through a simplify method. With the addition of UPFC in series compensation system which would lead to SSO, the equivalent impedance of the system changes. This change makes the electrical damping improved which can effectively suppress the SSO. Furthermore the influence of the operating condition on the electrical damping is qualitatively studied. The analysis shows that the change of the line active power would affect the system damping characteristics among the main control functions of UPFC.