Wet grinding and soapless emulsion polymerization of methyl methacrylate in an aqueous medium were simultaneously carried out in a vibrating rod mull. The usefulness of this process to modify fine powder surfaces was discussed in light of the characterization of the resulting composite powder. The inorganic powder used was quartz, and the initiator used in polymerization was a redox system of sodium bisulfite and potassium persulfate. By selecting the appropriate conditions, the surface of the ground quartz was covered with the formed polymer particles and was satisfactorily modified from hydrophilic to hydrophobic. The present process was found to be a useful method capable of surface modification and fine grinding of powder simultaneously.