Single atom catalysis involving atomically dispersed metal active sites on the appropriate supports is the effective way to magnify the catalytic efficiency and reduce the cost. By performing the first-principles calculations, we studied the anchoring of 3d transitionalmetal single atoms M (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) on the surfaces of MXenes Cr2CO2 and Mo2CO2 and the catalytic activity of the single atom sites for hydrogen evolution reaction (HER). Sixteen single atom sites, M-Cr2CO2 (M = Sc, Ti, V, Cr, Mn, Fe, Co, Cu and Zn) and M-Mo2CO2 (M = Sc, Ti, V, Cr, Mn, Fe and Zn) have been chosen via examining the energetical and thermal stability of the isolated M atoms on the substrates. More importantly, we have calculated the Gibbs free energy change (AG H ) of H adsorption on the surface of M anchored Cr2CO2 and Mo2CO2 and find that Cr, Fe, Zn on Cr2CO2 and Sc, V on Mo2CO2 are the promising single atom active sites toward HER. Additionally, our results show that M atoms adsorbing turns the nearby sites to be active for catalyzing HER. MXenes Cr2CO2 and Mo2CO2, in terms of the supporting not only stabilize but also works together with the anchored single atom M as active catalyst toward HER. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.