In order to solve the optimal coordination problem of directional relays in a complicated multi-loop power networks,a new model for directly obtaining MBPS（Minimum Break Point Set ）was established through the primary/backup relay’s coordination dependency relationships. Based on the ACO (Ant Colony Optimization) algorithm, a new algorithm was presented to solve the model. The formula of calculating the heuristic information was constructed to suit the MBPS problem.Compared to the available methods, the determination of the all directed loops of networks was completely avoided. More optimal solutions could be achieved at the same time after once algorithm calculation, and a smaller number of the iteration was required. The method was validated by the example.
