A new objective function (OF) has been proposed for mathematical formulation of directional overcurrent (OC) relay coordination in interconnected networks. The fuzzy-based genetic algorithm (GA) is applied to optimize the proposed OF for optimal coordination of OC relays. The defined fuzzy rules update the weighting factors of OF during the simulation. The miscoordination problem of OC relays is solved while decreasing the operating time of the relays. The proposed method is implemented in three different networks and the simulation results have been compared with previous studies in order to illustrate the accuracy and efficiency of the proposed method to coordinate the directional OC relays with both discrete and continuous time setting multipliers. The results have also been compared with the results of other optimization methods. Considering the coefficients of OC characteristic curves as the OF variables ensures that the proposed method has no limitation for the types of characteristic curves which will be utilized. Presenting the new term in OF, the performance of the proposed method has not been affected by the size of the networks.