Colloid mill attrition can remove the oxidized layer from oxidized coal surface. The effects of colloid mill gap space and milled pulp density were investigated according to the flotation response. It was found that both particle size reduction and flotation recovery enhancement were higher at higher pulp density and smaller gap space. When colloid mill worked at higher pulp density and smaller gap space, oxidized layers of coal particles could be removed more thoroughly. In addition, the greater reduction in particle size at higher pulp density and smaller gap space should be conductive to the flotation of oxidized coal because large particle size of oxidized coal is considered not suitable for its flotation. Because colloid mill was successfully applied in improving oxidized coal flotation, the results of this paper will promote the industrial application of colloid mill into effective flotation of oxidized coal in coal preparation plants.