In order to analyze the grinding behavior of toner sheets quantitatively, the relationship between Vickers hardness, Hv, and the mean particle size after grinding or the grinding throughput rate, Q, was measured and discussed. Toner sheet samples of different hardness and mechanical properties were prepared from polymer resin and carbon pigment particles by mixing, melting, and cold rolling. Toner powder was prepared from toner sheets of different hardness in a turbo or jet mill under various grinding conditions. The mean toner particle diameter after grinding under the same grinding condition increased with Hv in the case of the turbo mill. In the case of the jet mill, a strong correlation was indicated between Hv and Q when the mean particle diameter of the toner powder achieved was about 7 mu m. This relationship is expressed as follows. Q = 36Hv(-0.72)-1.15 Since the slope of the relationship between the grinding behavior and the maximum hardness was greater than that between the grinding behavior and the minimum hardness, it was suggested that the grinding behavior depended on the hard part in the toner sheets. The internal structure of the toner sheet was evaluated using a supersonic observation system; Hv was observed to be small when many detailed voids existed in the toner sheet. (C) 2006 Elsevier B.V. All rights reserved.