In the present paper, a theoretical examination is made of the retentate recycle to the permeate stream, which was verified experimentally as an effective method for enriching the less permeable component in the retenate stream using hollow fiber membrane permeators. The concentration of the less permeable gas component in the retentate was calculated as a function of membrane area, cut, pressure ratio, ideal separation factor, and the retentate recycle ratio. The concentration of the less permeable gas in the retentate had a maximum point as a function of the recycle ratio at a given operating condition (constant membrane area and pressure ratio). The recycle was more effective for obtaining high purity of the less permeable component when a more selective membrane (high separation factor) rather than a less selective one was used. It was also more effective with a large pressure ratio than with a small pressure ratio. The evaluation of the separation efficiency by Rony’s extent of separation and the tails separation factor also indicated that the case with the retentate recycle was more effective for obtaining high purity less permeable component than the case without the recycle, especially for a highly selective membrane and high pressure ratio. On the other hand, the retentate recycle to the feed was found to have no effect in improving the concentration in this calculation range.