This study presents a convex formulation for generation-transmission expansion planning in the presence of phase shifter transformers and aims at maximizing social welfare. By changing the voltage angle, the phase shifter transformer can control the transmission power of the line. Therefore, by installing a phase shifter transformer, one can reduce the investment cost of new lines and use the nominal capacity of available transmission lines. Accordingly, the planning of the generation-transmission expansion in this paper is formulated with the assumption that there is a pool electricity market. The problem is formulated in the form of mixed integer programming and the CPLEX solver is used to solve it in the YALMIP Toolbox environment. In the proposed method, the location, capacity and year of installation of generators, transmission lines and new phase shifter transformers will be determined simultaneously. In order to validate the proposed planning, the expansion planning of the IEEE 24-bus system has been simulated in MATLAB software. Simulation results show the efficiency of the proposed method