Three different kinds of Al2O3 have been added to HZSM-5 for improving its activity towards selective catalytic reduction of NO with CH4 in the presence of excess of oxygen. Higher conversions of NO have been observed for all of the three mixed catalysts, the conversion of which were also found to be higher than that of each separate component of the catalysts. It was concluded that there exists a synergetic effect between Al2O3 and HZSM-5. The addition of Al2O3 can improve the catalytic activity of HZSM-5 by inhibiting the decomposing of NO2 to NO and accelerating the activation of methane. Adsorbed NO2 is then reduced by the activated methane or the intermediates formed from activated methane to N2 over the Brønsted site of zeolite.