Abstract: A novel unsteady-state membrane permeation process was proposed based on steady-state permeation. The process is composed of three basic cyclic steps:feed pressurized, permeate withdrawn, and residue vented. The effects of pressurization time, evacuation time and venting time on the average purity, productivity and recovery of nitrogen were experimentally investigated for air separation to produce nitrogen by this process. The effectiveness of the process was compared with that of steady-state permeation. The results show that the average purity, productivity and recovery of nitrogen increase with the prolonging of pressurization time under the experimental conditions. The average purity of nitrogen-enriched gas is increased but its productivity and recovery are decreased when the evacuation time is extended. An appropriate increase in venting time is associated with the increases of average purity, productivity and recovery of nitrogen. When the venting time is too long, the productivity and recovery will be decreased. As a result, the proposed process in this paper can achieve a higher purity of nitrogen but a lower productivity and recovery.