Abstract: A pore-distribution model was developed for predicting and evaluating gas separating property of polyvinyl dimethylsiloxane-polyvinylidene fluoride (PVDMS-PVDF) composite hollow fiber membranes. The experimental data of both permeability and selectivity are in good agreement with the theoretical results predicted by the pore-distribution model. The PVDMS-PVDF composite hollow fiber membranes were prepared through dry-wet phase inversion and dip-coating technique, of which the structural parameters of PVDF substrate membrane were estimated by gas permeation test, and the separating property was evaluated experimentally using N2/O2 as the medium. The experimental results indicate that the composite membrane has high selectivity/separating property and the thickness of PVDMS skin coat must be higher than 5 pm. so as to be compact without defects.