After Optimal Distribution Substation locating, distribution feeder routing is the main problem in distribution system planning and its expansion. This paper presents a new approach based on simultaneous application of graph theory and genetic algorithm to solve optimal high voltage substation placement and feeder routing in distribution system. The proposed method solves hard satisfactory optimization problem with different kinds of operational and optimization constraints. Since it is formulated as a combinatorial optimization problem, it is difficult to solve such a large scale problem. A minimum spanning tree algorithm is used to generate a set of feasible initial population. To reduce computational time and avoiding from infeasible solution a special coding is generated for GA operator such as crossover and mutation. This coding guaranties validity of solution toward global optimum. The method is examined in two large – scale distribution system.