Size dependence of C-2 oxygenate formation from syngas on Cu cluster has been investigated to qualitatively probe into the effect of Cu cluster size on the selectivity of C-2 oxygenates, which includes two key steps: CHx and C-2 oxygenate formations; Cu-13, Cu-38 and Cu-55 clusters have been employed to model different sizes of Cu cluster. Here, density functional theory method has been performed. Our results show that the adsorption ability of the species involving in C-2 oxygenate formation decreases with the increasing of Cu cluster size; Cu cluster size significantly affects the dominant existence forms of CHx(x=1-3) species and C-2 oxygenates. Among three Cu clusters, Cu-13 cluster exhibits the highest selectivity toward C-2 oxygenates compared to other two clusters, suggesting that Cu cluster size can affect the selectivity toward C-2 oxygenates, moreover, the smaller Cu cluster size is, the higher the selectivity of C-2 oxygenates is. That is probably related to the high concentration of low-coordinated defect sites on small size Cu cluster, which results in the higher activity and selectivity toward C-2 oxygenates. The identification of higher intrinsic selectivity of C-2 oxygenates, active sites, and stronger cluster size effect would be valuable for developing more efficient and stable Cu catalyst with higher selectivity toward C-2 oxygenate in syngas conversion. (C) 2017 Elsevier B.V. All rights reserved.