The mass transfer for dialysis coupled with ultrafiltration through a cross-flow rectangular membrane module has been studied. Theoretical analysis of mass transfer in cross-flow membrane dialyzers was analogous to heat transfer in cross-flow heat exchangers with the assumption that the concentration variation in the cross-sections of flow channel was negligible. The simpler but still precise equation for predicting the total mass transfer rate might be more powerful than the exact solution obtained in previous work to overcome the mathematical difficulties in device design, especially for low flow rate of retentate phase.