In this paper, the effect of the islands number variation in the restoration process is investigated and a method to determine optimum island boundaries is presented. Also, the impact of Parallel Reactive Power Sources (PRPS) on the number and boundaries of each island and restoration process is studied. In the optimization process, the objective function is minimizing Energy Not Supplied (ENS) and constraints are voltage margins in buses, transmission lines capacities and generators loading limits. The ENS is evaluated based on generation capacity allocation to demand loads method. Genetic algorithm (GA) is the base of optimization and an appropriate chromosome coding is developed for the network modeling. In order to assess the capabilities of the proposed method, the IEEE-118 bus network has been used as the test system. The results of sectionalizing the test system to 2 to 7 islands with optimum boundaries have been given in case studies.