This study deals with the effect of an exponential-decay flux of ultrafiltration on the mass transfer rate of solute for dialysis in cross-flow membrane modules. The numerical method of orthogonal collocation was used to solve this problem. The calculated results of solute concentrations and separation efficiencies were analyzed under various operating conditions and decay coefficients of nux in ultrafiltration, The mass transfer rate is usually enhanced by the addition of the effect of ultrafiltration, but this effect decreases as the decay coefficient increases. A significant influence of the decay coefficient on the mass transfer rate appears when the solute transport is dominated by the effect of ultrafiltration, i.e. the conditions of lower dialysate flow rate, lower solute sieving coefficient, or loner overall mass transfer coefficient.