The permeation behavior of pure He and separation characteristics of He-air mixtures were examined using asymmetric hollow-fiber-type membrane modules of cellulose triacetate. The module was operated in a feed-outside mode. In permeation of pure He, the permeation rate coefficients slightly increased with increasing upstream pressure, and they agreed fairly well with those calculated on the basis of the assumption that the permeate flow inside the fiber is governed by the Hagen-Poiseuille equation. In the single-stage hollow-fiber module used, the molar fraction of He in the permeate stream was increased to 0.99 and 0.999 at the stage cuts of lower than 0.3 when the molar fraction in the feed stream was 0.90 and 0.99, respectively. The molar fractions of He in the permeate stream were numerically evaluated for various combinations of operating conditions in terms of a countercurrent plug flow model.