Dry and wet grinding of EP dust containing mainly carbon and ammonium sulfate with small amounts of metal compounds were carried out by using three types of grinding mills. The characteristics of ground specimen and solubility of metal compounds in distilled water were investigated by XRD, EPMA, SEM, particle size distribution measurement and ICP analysis. Aggregation of fine particles occured in dry grinding with increase in grinding time, while continuous size reduction of the ground specimen occured in wet grinding. Dry grinding of EP dust causes structural change from crystalline metal compounds to amorphous ones. The water solubility of vanadium compound in the specimen ground under dry condition increases with an increase in grinding time, while that of nickel and iron compounds are almost independent of grinding time. The maximum concentrations of vanadium are about five to six times higher than those for the original unground specimen. Wet grinding does not contribute to the improvement in solubility of metal compounds.