—This paper proposes the energy function of a power system with a Static Synchronous Series Compensator (SSSC). They make it possible for the direct method to acquire the transient stability or critical clearing time assessment of a power system. The proposed energy function of a power system with a SSSC is first derived and then it is used for SSSC control design. The proposed method is tested on the single machine infinite bus (SMIB) and multimachine system.