Au/ZnO catalysts prepared by co-precipitation method with different Au:Zn atomic ratios have been studied for room-temperature CO oxidation in the presence/absence of water in the feed stream. The catalysts exhibited excellent catalytic activity and stability compared with pure ZnO. Furthermore, it was found that the lifetime of Au/ZnO catalysts was improved significantly by increasing CO concentration but decreased by addition of H2O in the feed stream. The deactivation may be related to the sintering of metallic gold and the accumulation of carbonate-like species in the catalyst.