The Au/SnO2 catalysts were prepared by a co-precipitation method. The structure of the sample treated at different temperatures was investigated by means of HRTEM and XPS. The structure of the samples after various treatments and their activity in the CO oxidation were compared. The results showed that the catalytic behavior was related to the particle size of gold and surface oxygen species on the support. Highly dispersed gold particles and adsorbed surface oxygens and hydroxyl groups on the support were responsible for the high catalytic activity of the Au/SnO2 catalyst.