Density functional theory (DFT) calculations were carried out to investigate the reaction mechanism of ethanol steam reforming (ESR) reaction on TM/Mo6S8 (TM = Rh, Ir) clusters. We investigated the reaction mechanism of TM/Mo6S8 catalyzed ESR by studying the cleavage of C-H, C-C, and O-H bonds and the formation of hydrogen, and explored the catalytic activity of TM/Mo6S8 in order to find superior catalyst. Our results indicate that ESR is first decomposed by ethanol and then subjected to water-gas shift (WGS) reaction to produce H-2 and CO2. In addition, we found that the formation of CH4 and CO is favored as the products of ethanol decomposition, the O-H bond cleavage of OH* is considered as the key step in ESR. According to our calculations, we found that Ir/Mo6S8 was more favorable for catalyzing ethanol decomposition and Rh/Mo6S8 was beneficial for catalyzing WGS. However, for the whole reaction, Rh/Mo6S8 exhibits better catalytic performance than Ir/Mo6S8 because of low energy barrier of rate-determining step. A comparison of micro-kinetic modeling, the metals d-band and projected density of state (DOS) show that Rh/Mo6S8 is superior catalyst. This approach provides theoretical insight into the reaction mechanism for suppressing the carbon formation on TM/Mo6S8 (TM = Rh, Ir) and is expected to have a significant implication on general methods of the ESR catalyst design in order to have better activity and stability. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.