To increase the co-firing ratio of biomass in existing pulverized coal-fired power plants, coal and biomass should be pulverized to a suitable particle size for combustion. In this study, the grinding characteristics of coal mixtures with different wood pellet (WP) ratios were evaluated using a bench-scale roller mill. The power consumption and differential pressure of the roller mill increased with increasing WP mixing ratios. When grinding the mixtures composed of coal and WPs, the wood particles were selectively retained and these particles accumulated inside the roller mill. As the WP mixing ratio increased, the wood particle size in the grinding mixture also increased. Many of the large particles were composed of wood. Furthermore, the proportions of small coal particles decreased, and those of large coal particles increased as the WP mixing ratio increased. Our results indicated that the mixing ratio of WPs significantly affected the operating parameters of the mill and the particle sizes of coal and biomass in the grinding product.