Abstract: A series of CeO₂ catalysts was modified with different acids including phosphotungstic acid (P-W), phosphate acid (P), partial ammonium tungstate (W), and phosphate acid plus partial ammonium tungstate (P+W); the effect of acid modification on the performance of CeO₂ catalyst in the selective catalytic reduction of NO with ammonia (NH₃-SCR) was then investigated. The results indicated that the CeO₂ catalyst after acid modification possesses abundant surface acid sites of weak-medium strength and the acid quantity of various catalysts follows the order of P-W/CeO₂ >W/CeO₂ >P+W/CeO₂ >P/CeO₂. Due to the interaction between P and W species, the P-W/CeO₂ catalyst modified with phosphotungstic acid exhibits high amount of weak-medium surface acid sites and active Ce and O species, which may promote the adsorption and activation of NH₃ and then enhance its catalytic activity in the NH₃-SCR of NO; over the P-W/CeO₂ catalyst, the conversion of NO keeps above 90% at 225-450℃.