The effect of Pd-modification of Co/HZSM-5 on durability for NO removal was studied. The durability of catalysts subjected to hydrothermal treatment with a He stream containing 10% steam at 600degreesC for 24 h was investigated. The loss of catalytic activity for NO conversion after hydrothermal treatment on Co/HZSM-5 was due to framework dealumination of the zeolite. The Pd-modification of Co/HZSM-5 resulted in improved durability against the hydrothermal treatment when loading Pd approximately 0.4 %wt. From NMR spectra, dealumination of tetrahedral aluminum in the zeolite framework was completely prevented when loading Pd from 0.4 %wt or higher.