Due to increasing demand of power systems, which results from technology development, population growth, and ever-increasing need for electrical energy, it is necessary to expand the transmission network with the proper construction of new lines, so that the network can handle future forecasted loads. These studies are called transmission expansion planning (TEP). High amount of investment is required for TEP and so it is very important to find the optimal transmission plan that minimizes this cost. To find such a plan, an accurate and relatively complete model of the problem should be used in which different technical and economical constraints are considered. In recent years, efficient efforts have been made to improve the TEP. Some of these efforts are construction of new lines in several steps (multi-stage TEP), considering security constraints and reliability criteria, and including load uncertainty and transmission losses and so on in transmission expansion planning problem. In this thesis, consequences of different simplifications in initial models are firstly investigated by reviewing and simulating some of the more complete models presented in recent years. For this sake, some features of the PSO algorithm such as high convergence speed, good performance and flexibility are very useful to solve the problem and achieve this goal especially in more complicated models. One of the major issues in the power system expansion planning problem is the mutual impact of TEP and generation expansion planning (GEP). Conventionally, these two problems are solved independently, and no coordination is established between them. In this thesis, to coordinate TEP and GEP, GEP is firstly solved as a single-bus model, and the new power plants which have to be added to the system are identified. Then the generation capacities of the new power plants are considered to be within a flexible range around the amount calculated by GEP, and the transmission expansion planning problem is solved. This results a lower investment cost with a new generation capacity for new power plants in the predefined flexible range. Reduction in the total investment cost of TEP and GEP compared to the common expansion planning procedures which consider a certain specified amount for the new generation capacity, verifies the performance of the proposed method and the creation of proper coordination between TEP and GEP. Finally, the calculations are repeated by gradual changing of the boundaries of the flexible range and highest coordination and lowest amount for the total investment cost is obtained. Simulation of the proposed method in a test system shows its capabilities and advantages compared to the conventional methods. Keywords : Transmission expansion planning, Generation expansion planning, Coordination of TEP and GEP, PSO algorithm