In this paper, we propose a model for the generation and transmission network expansion planning problem that includes decisions related to substations’ locations and sizes. In power system expansion planning problems, locations of generation units and/or transmission lines are determined. However, substation location decisions are not explicitly studied. Nevertheless, including the decisions of substations’ locations and sizes to the problem can change the transmission network substantially. Hence, in this study, we first discuss the benefit of incorporating substation decisions to the transmission network design problem. We then propose a model that finds a minimum cost network, the optimal locations and sizes of substations and generation plants. To overcome the computational challenge of the proposed model, we present a heuristic solution approach, in which we decompose the multi-period problem into several single-period problems that are solved sequentially. Computational studies on 24-bus and 118-bus power systems show that the proposed solution approach yields near optimal solutions in minutes.