This paper presents an optimization model for calculating the available transfer capability（ATC） of hybrid AC/DC systems. The hybrid systems can include either two-terminal DC systems or multi-terminal DC systems. The nonlinear primal-dual interior point method is used to solve this model. To increase calculation efficiency， a reduced correction matrix is derived. By exchanging its rows and columns， a data structure convenient for storing and writing programs is constructed. A modified IEEE 30-bus hybrid system is used as test case. Simulation results show that the proposed model is feasible， and that the ATC can be enhanced via reactive power compensation at the converter station.