Abstarct: The electricity distribution network is one of the important parts in power systems and delivering quality power to the customer's needs a suitable design of the distribution network. Optimal placement of distribution substations and feeders which is abbreviated by "distribution system planning" in this thesis, is also one of the vital issues in the distribution network. As the cost of distribution network is a part of the total cost of the power system, the distribution network is designed with the aim of minimizing the cost. The aim of this thesis is to perform the distribution electricity network planning with using branch exchange method and distribution load flow considering the presence of the distributed generation. The objective function in this thesis includes installation cost, operation cost and location of feeder and substation. The constraints of the optimization problem are also substation capacity, feeder thermal capacity, voltage drop and configuration of the distribution network. The distribution system planning is firstly undertaken for the case when there is no distributed generation resources. In this case, the optimal placement of feeders and substations is found using intrazone and interzone branch exchanges and structured power flow. In the second case, distribution system planning is performed when the distributed generation resources are present. In this case, the optimization technique employs genetic algorithm. The algorithm for both cases are finally applied to a test distribution network to find the optimal placement of the feeders and substations. The results of the simulations reveal that the algorithm presented in this thesis are capable in determining the optimal location of the feeders and substations with and without distributed generation resources.