Presence of Distributed Generation (DG) in electric distribution systems provides an attractive new alternative in capacity expansion planning of these systems. In this paper a new model is developed for solving simultaneous placement of HV/MV substations and DG units as a static planning tool. This model aims to minimize fixed and variable costs of planning. Constraints related to DGs, substation and network are incorporated in the developed model. Proposed model can be used for optimal sizing, sitting and defining associated service area of HV/MV substations and DGs in distribution capacity expansion. The proposed model is a mixed-integer nonlinear mathematical model which optimized using genetic algorithm. The effectiveness of proposed model is illustrated through numerical examples.