Room temperature grinding of four kinds of powder materials chosen based on their grindability was conducted using a planetary ball mill, in which the powder is charged in different amounts besides media balls. The median diameter of the ground materials was monitored to experimentally determine the size reduction rate. The size reduction rate of each material is proportional to specific impact energy of the balls simulated by the PEM (or called DEM). The gradient of the straight lines for each material in the prescribed relation is correlated to a net work index, which is calculated by substituting the impact energy of balls into the term of energy in Bond＇s equation. The relation between gradient and net work index is also expressed by a straight line in normal coordinates. Therefore, the size reduction rate can be estimated by introducing a specific impact energy simulated by the PEM.