Variations in the grindability of semi-cokes produced by two low-rank coals at different pyrolysis temperatures were studied. The grindability of semi-cokes prepared by Ximeng lignite (XL) had,minimal changes at varying pyrolysis temperatures and was nearly similar to that of a dehydrated sample. The pore structure of XL was fully developed after dehydration and had no significant change during pyrolysis, as indicated by the observations obtained through scanning electron microscopy and results of the mercury intrusion porosimetry experiment. Thus, its grindability showed little change during pyrolysis. Meanwhile, the grindability of Shenhua sub-bituminous coal (SC) decreased considerably when the pyrolysis temperature reached 550 degrees C. As shown by the results of thermogravimetric experiments, X-ray diffraction, and fractal analysis of the pore structure, the coal matrix of SC had a plastic stage. Meanwhile, its volatile matter rapidly releases in this stage. After the plastic stage, considerable changes occur in the pore and chemical structures of SC. All of these factors eventually altered the grindability of SC.