The adsorption characteristics of nitrogen and oxygen on acid-treated and cation-exchanged Korean natural zeolite（mordenite type） was investigated. Pore volume of natural zeolite increased, whereas adsorption amount of nitrogen decreased with the treatment of acid. When metal ion of natural zeolite was exchanged with Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺, the adsorption amount of nitrogen increased with increasing cation size. Although a similar tendency was observed in alkali metal ion exchange, i.e., the Na⁺-exchanged natural zeolite adsorbed more nitrogen than the Li⁺-exchanged, the K⁺-exchanged resulted in decrease of nitrogen adsorption due to large decrease in pore volume. With a separation factor of 8.9 and adsorption amount of 7.4 ml/g nitrogen(at P_N2=610 torr), the Na⁺-exchanged natural zeolite brought a good result as an adsorbent to be used in separation of nitrogen and oxygen.