To elucidate structure-reactivity relationship and promote the reaction performance of SnO2/Al2O3 for NOx-SCR, a series of catalysts have been prepared by impregnation method and treated by non-thermal dielectric barrier discharge (DBD) plasma. It is quantified that SnO2 disperses on gamma-Al2O3 with a monolayer capacity of 0.262 mmol 100 m(-2). The catalyst with a SnO2 loading close to the capacity displays the best activity, testifying the presence of a monolayer threshold effect. Plasma treating can improve SnO2 dispersion evidently, thus expanding the monolayer capacity to 0.340 mmol 100 m(-2). As a result, the surface Sn cations are enriched, which improves the surface acidity of the catalysts, benefits the adsorption and activation of NO and C3H6 molecules, and eventually enhances the activity. It is concluded that the surface acidic site amount determines the reaction activity of catalysts. Over SnO2/Al2O3 catalysts, NOx-SCR by C3H6 follows a Langmuir-Hinshelwood mechanism. Surface monodentate nitrate and formate are found to be the major active intermediates for the reaction.