The characteristics of particle size reduction of Korean anthracite ash (250-300, 300-425, 425600, and 600-710 mum) by a gas jet in an ASTM attrition tester (D5757-95) with four different orifice sizes (0.38, 0.7, 1, and 1.5 mm) have been determined. The particle size reduction rates of anthracite ash were found to be a function of the operating parameters (superficial gas velocity, orifice gas velocity, bed weight) and the physical properties of the solid and gas phases (particle size and gas density). A simple model for particle size reduction rate based on the single-orifice attrition theory is proposed. The particle size reduction rate is proportional to the orifice velocity and gas density.