The mass transfer for membrane dialysis through a flat-plate module has been studied theoretically with or without the external reflux, which is analogous to the heat transfer in a flat-plate heat exchanger. Experiments were carried out by using the membrane sheet made of cellulose ester as a permeable barrier to dialyze urea from the aqueous solution. Theoretical predictions are in good agreement with the experimental results. In contrast to the device without recycle, considerable improvement in mass transfer is obtainable, if the membrane dialysis is operated with recycle. The recycle can enhance mass transfer, especially for operations with higher volumetric flow rate, higher inlet urea concentration or reflux ratio.