The 4step-2bed process(Process 1) using Skarstrom cycle, 5step-2bed process(Process 2) using product-pressurization and purge step, 3step-3bed process(Process 3) using product-pressurization, and 6step-2bed processes(Processes 4, 5, and 6) using pressure-equalization were compared in terms of recovery and productivity versus oxygen purity. Feasible operation region was studied with relation of feed velocity and pressure. Process 2 yielded the highest productivity in the feasible operation region. However, only low purity product was obtained. For high purity, Process 3 was mostly the best. As for recovery, Process 4 including pressure-equalization step yielded the highest one. For air PSA, flow direction from product-end to product-end during pressure-equalization step was most desirable. For all processes, recovery increased with pressure in the low pressure range and decreased above pressure ratio of five, indicating that the optimum operating pressure existed.