Abstract: The stability and catalytic activity of Au₁₀, Au₁₃ and Au₂₀ clusters in water gas shift reaction (WGSR) were investigated by density functional theory (DFT); the adsorption behavior of reaction species and the reaction mechanism of WGSR on various Au_n clusters were explored. The results indicated that the stability of three Au_n clusters follows the order Au₁₀ < Au₁₃ < Au₂₀, whereas their electron delocalization and adsorption capacity decreases in the sequence of Au₁₃ > Au₁₀ > Au₂₀. Three Au_n clusters exhibit the same rate-determining step for WGSR, i.e. H₂O dissociation; however, they are quite different in the actual reaction routes. Over Au₁₀ cluster, the WGSR reaction follows the carboxyl mechanism, characterized by the direct dissociation of COOH^*; over Au₁₃ cluster, the redox mechanism applies, suggested by the disproportionation of two OH^*; over Au₂₀ cluster, the WGSR reaction proceeds via the carboxyl mechanism, represented by the disproportionation of COOH^* and OH^*. A comparison for the optimal reaction paths over three Au_n clusters suggests that the Au₁₃ cluster has the highest catalytic activity in the WGSR reaction at low temperature.