Non‐steady‐state flux ratios of oxygen to nitrogen in a poly(dimethyl siloxane) membrane are theoretically investigated as a function of time. Concentration profiles of oxygen and nitrogen are found to decay to a negligible concentration with an increase of the distance from membrane surface and it is observed that oxygen penetrates deeper than nitrogen in the membrane at the non‐steady state. Differences of concentration profiles of oxygen and nitrogen lead to enormously high selectivity for oxygen at the non‐steady state. The flux and selectives are studied under the condition that the upstream pressure is pulsating to keep the non‐steady state continuously as suggested by Paul[Ind. Eng. Chem. Proc. Des. Dev., 10, 375 (1971)]. Model calculations are also performed for the concentration of uranium 235 from natural uranium. The flux ratio of uranium 235 to uranium 238 in the non‐steady state is calculated to be 1.144 at time/L2 = 800 s/cm2 (L = membrane thickness), which is higher than the value, 1.00429, at a steady state.