To obtain the reasonable starting points for setting directional protections in loop grid, the method for selecting minimum break point set is studied based on the coordination performance of protections. According to the step principle for setting protection relay, the measurement index is defined to evaluate the coordination performance of directional protections of transmission lines in loop grid, and the effectiveness of each feasible solution of minimum break point set problem can be described by its measurement index value, and the feasible solution with the best measurement index can be further selected as the minimum break point set. Based on the concept of the longest path in graph theory, the method for calculating measurement index is also given. Simulation results show that the proposed method can avoid the inappropriate coordination of protections caused by the unreasonable selection of minimum break point set.