Abstarct: In this thesis, modified bat algorithm is used to optimize the annual cost and reducing the losses in the electrical power distribution systems. The designed algorithm minimizes the losses of power network lines in different conditions. The purpose is to use modified bat algorithms to reduce annual costs and energy losses in power distribution systems. Reducing these loses have efficiency in the electricity companies. The proposed method is simulated on two networks of 34 and 69 bus IEEE test system. In the used method, the capacitor values and convergence is examined. Obtained results of modified bat algorithm are compared to other optimization methods. The results show the performance of this method.