Abstract: The gas permeability of compacted GMZ bentonite is studied using the triaxial gas permeameter. It is found that the Klinkenberg effect exists when the gas permeability is lower than 10^-14 m². A power relationship is observed between the Klinkenberg slippage factor and the ratio of the Klinkenberg permeability to the air-filled porosity. The relationship is independent on dry density and water content, and is almost identical with the Klinkenberg model. In the double-log scales, there is a linear relationship between the Klinkenberg permeability and the air-filled porosity, of which the slope is almost independent on water content. Based on the test results, a model for unsaturated gas permeability is proposed, in which the Klinkenberg effect is taken into account.