A series of Pt/Mg-Al-O catalysts with different Mg/Al atomic ratios were prepared. The NO_x storage capacities of these catalysts were measured by isothermal storage at 350 ℃. It was found that the NO_x storage capacity increased with increasing Mg/Al atomic ratios. The catalytic behaviors of Pt/Mg-Al-O and Pt/MgO were studied with storage-reduction cycles at 400 ℃. Under oxidizing conditions, NO_x concentration in the outlet gas gradually increased with time, which indicated the catalysts could store NO_x effectively. After a switch from oxidizing conditions to reducing conditions, NO_x desorption peak emerged immediately due to the incomplete reduction of stored NO_x, which lowered the total NO_x conversion. With increasing Mg/Al atomic ratio in the catalysts, NO_x conversion increases. Pt/MgO has the highest NO_x conversion because of its best activity in the reduction of NO_x by C₃H₆. It seems that with an increasing amount of MgO in the catalysts, the self-poisoning of Pt-sites by adsorbed species during the reaction of NO_x with C₃H₆ may be inhibited effectively.