The mass transfer for membrane extraction through a cross-flow parallel-plate module has been studied both theoretically and experimentally. Theoretical analysis of mass transfer in cross-flow membrane extractors 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. Experiments were carried out with the use of membrane sheet made of microporous polypropylene coated with polytetrafluoroethylene as a permeable barrier to extract acetic acid from aqueous solution by methyl isobutyl ketone. Theoretical predictions are in agreement with the experimental results. The simpler but still precise equation for predicting the total mass transfer rate might be more powerful than the exact solution obtained in the previous work to overcome the mathematical difficulties in device design.