Abstract: To build a theoretical framework of the best grid size of occupancy grid map, a cost function with correctness and information amount as variables is put forward to evaluate the accuracy of occupancy grid map. As a result, the best grid size can be figured out by the cost function. Significant map rate is presented for the first time to evaluate the correctness of an occupancy grid map. Through mathematical derivation, its theoretical calculation approach is obtained to build the relationship with the grid size and sensor accuracy. Also, the total number of grids is used to characterize the information amount. Simulation results show that the theoretical calculation method of significant map rate is valid. Meanwhile, the scanning experiments with RplidarA2 radar show that the proposed calculation method of the best grid size is correct, demonstrating that the significant map rate is a characteristic parameter of a map.