Abstract: When running a hammer mill the traditional method is to take the rated power of the electric motor to be selected as the optimum load of that hammer mill. An experiment was carried out to test the relationship between the load and following characteristics: different screen meshes, the load of the hammer mill, the productivity of the hammer mill, and the electric power consumption per ton of hammer mill product. The result shows that the maximum productivity and the minimum electric power consumption are all deviated from the rated power of the electric motor. Meanwhile, the smaller the sieve mesh, the greater the deviation. The conclusion of the experiment is that the optimum load of a hammer mill should be determined by the load test. By the load tests, author derived a regression equation for the rclationip between the productivity of the hammer mill and the electric power consumption per ton of the hammer mill product. At the same time, author also presents the method to find out the lowest cost per ton of product with the optimum load of the hammer mill in order to get the running economy of the hammer mill and to provide the basis for automatic control.