Recently the authors proposed the following experimental formulas expressing the zero order increasing rate constant of mass fraction less than a size x, Kx, during dry and wet ball mill grindings.Dry: Kx(D)=0.019HV-0.47(WS/WB)-1.5(x/xf)(xf/xo)0.7Wet: Kx(W)=0.05HV-0.47(WS/WB)-1.0(x/xf)(xf/xo)0.5On the other hand, Work Index Wi proposed by Bond was inversely proportional to the ball mill grindability Gb·p0.82 and Gb·p was proportional to Kx·WS.From these results, it was found that the difference of Work Index Wi between dry and wet grindings was expressible by the following equation.Wi(W)/Wi(D)={0.38(WS/WB)-0.5(xf/x0)0.2}0.82