This study proposes a model based on mixed-integer linear programming (MILP) for the integrated expansion planning of generation and transmission systems with the implementation of distributed generation (DG). Most DG planning takes place after generation and transmission planning has been conducted. This model can be used to include DG potential simultaneously with generation and transmission expansion. DG is modelled as a negative load therefore DG is treated as a non-dispatchable unit of power generation. The objective of the model is to minimize overall cost including the investment cost of the generation units, DG units, and transmission lines, and the operating cost of the generation and DG units. The proposed model is static-deterministic model in the form of MILP. The model was evaluated using the 6-bus Garver’s test. To prove the effectiveness of the model, it was evaluated using the IEEE 46 Bus Test. The results show that due to the impact of DG on power system expansion planning ,the overall cost was reduced. The simulation results also show that a different optimal network configuration can be achieved by DG implementation in expansion planning.