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The effects of grain size, space velocity, temperature and reactant concentration on the kinetics of NOx reduction with propane over Co-β-zeolite catalyst were investigated. The external mass transfer phenomenon was examined by varying the space velocity. The results show that the transfer can be negligible when the space velocity is greater than 60000 h-1 in low temperature range. However, the transfer exists at high temperatures even when the space velocity reaches a high level.Variation of the catalyst grain size from 0.05 to 0.125 mm does not change the conversion rate of NOx. The concentrations of components, NOx, C3H8 and O2, were also investigated to have a better understanding of mechanism. Based on the experimental data, the selectivity formula was proposed. The results shows that lower temperature is helpful to get higher selectivity as the activation energy of hydrocarbon oxidation, Ea,2, is greater than that of NOx reduction, Ea,1, (Ea,2＞Ea,1). High NOx concentration and low C3H8 concentration are beneficial to high selectivity. However in order to maintain high activity simultaneously, the temperature and C3H8 concentration should be high enough to promote NOx reduction. 10%(ψ) H2O and 75×10-6(ψ) SO2 were introduced into the reaction system, and Co-β-zeolite shows strong resistance to water and SO2.