The structural properties of Ag/SBA-15 catalysts with various Ag loading after pretreatment under different atmosphere, i.e., O-2, Ar and H-2, in relation to their activities on CO oxidation were investigated. N-2 adsorption-desorption results showed that the pretreatment atmosphere had a negligible effect on the physical structure of all catalysts. Interestingly, it was found that different pretreatment atmosphere (Ar, O-2) and Ag loading exhibited a large difference on the structure of silver catalyst and their catalytic activity. For Ag/SBA-15 catalyst with low silver loading (<= 6 wt%), Ar pretreatment resulted in the highest activity for CO oxidation. However, the highest catalytic performance was observed after oxygen pretreatment over the silver catalyst with high loading (>6 wt%). UV-vis and H-2-TPR results showed Ar pretreatment and Ag loading strongly affected the states of Ag species for the silver catalysts, which were changed from metal Ag (<= 4wt%) to a mixture of Ag and Ag2O (>= 6 wt%). Moreover Ar pretreatment was favorable for the formation of small Ag particles for Ag/SBA-15 catalyst with low Ag loading. When the silver loading is increased, the heavy aggregation of silver particles occurred after Ar pretreatment, resulting in the decrease of TOF for CO oxidation. O-2 pretreatment was conductive to the formation of subsurface oxygen species on the larger silver particles for Ag/SBA-15 catalyst with high Ag loading, which resulted in the increase of TOF for CO oxidation. It was evident that small metal Ag particles was key factor of the higher catalytic activity for Ag/SBA-15 catalyst with low Ag loading, while a large number of subsurface oxygen species played an important effect on the higher catalytic activity for Ag/SBA-15 catalyst with high Ag loading. (C) 2013 Elsevier B.V. All rights reserved.