Granulation is an operating process for solid treating in which powdered material grows into spherical granules of some sizes on a rotating disc by adding a certain amount of an appropriate binder and spraying water continuously. In this research a mathematical model for prediciting the sizes growing granules with time was proposed and its applicability was examined through a series of experiments. Adhesion coefficient D increased as the feed rate(F) of powdered graphite decreased and when feed rate(F) was fixed, the D values increased as the spraying rate of water(W) and rpm(N) of the disc increased. An empirical correlation between adhesion coefficient D and operating variables was obtained as follows, D=0.560×10^-3(W/F)^3.90N^1.50 Finally, the theoretical results of D from the proposed mathematical model were in good agreement with those from experiments and so the assumed model may wel be applicable to other systems of spherical growing ball.