The determination of minimum break point set（MBPS） for optimal coordination of directional relays in a multi-loop network is reduced to a 0-1 integer programming problem， where the heuristic knowledge of MBPS is facilitated. On the basis of the principle of ant colony optimization（ACO） algorithm， a new algorithm to determine MBPS is presented. The formula of calculating the heuristic information is constructed to suit to the MBPS problem， also the method of calculating the pheromone is improved. More optimal solutions can be got at the same time after once algorithm calculation， and a smaller number of the iteration is needed.