The mechanism of Ca²⁺ ion exchange in zeolite NaA powders were studied with varying its crystal size. It was reasoned that the rate to ion exchange at corners and edges of a crystal would be faster then that at the center portion of each crystal face. Therefore, as the degree of ion exchange advances, the front of ion exchange will lose its sharp edges and approaches to a near spherical shape. To take into account of this phenomenon in the analysis of experimental ion exchange rates, rate equations for sphere and cube were combined together in the following form, which may be called as the transition model. f_r(Φ)=F(Φ)/ ln Φ= [1-g(θ)] f_c(Φ) + g(θ) f_s(Φ) The transition time function, g(θ), was assumed to be expressed by g(θ) = αθ^βand the constants were found to be α=2.79, β=0.43 in this experiment. Using the transition model equation, the ion exchange rate of zeolite NaA powders would be represented better than either by the spherical or the cubic model alone.