Abstract: Available transfer capacity (ATC) is the key index to measure the security and stability of the interconnected power systems. The flexible AC transmission system (FACTS) is an effective means to improve the available transfer capacity. In order to improve ATC, static synchronous series compensator (SSSC) is stalled in the system. By using of the power injection method, the mathematical model of SSSC is established. Then the power injection model of SSSC is added in power flow equations based on optimal power flow method, and calculation model of ATC with SSSC is built, which is solved by primal-dual interior point method. The most suitable installation location is determined according to the sensitivity coefficients of the minimum modulus eigenvalue to control variables by using of eigenstructure analysis approach. Taking the IEEE\|30 bus system as example, the simulation results show that the transmission capacity and the stability margin are improved after the installation of SSSC, which indicates that suitable allocation of SSSC can improve the system available transfer capability effectively.