Selective hydrogenation of glucose into 1,2-propanediol (1,2-PDO) and ethylene glycol (EG) over supported Ni catalysts were investigated by C-13 NMR, XRD, TEM, and EDS, as well as first principles calculations to reveal the significant role of the support on the catalytic mechanism and the product selectivity. Our experiment shows that the MgO support facilitates the conversion of glucose into fructose and the retro-aldol reaction of fructose at the C3-C4( )position to form C-3 products. Density functional theory studies of the MgO support effect on the retro-aldol reactions of fructose and glucose indicate that the formation of C-3 products is preferred with lower energy barrier, consistent with our experimental observation. Possible reactions occurred during the hydrogenation of glucose over MgO and ZnO supported Ni catalysts were discussed based on this combined experimental and computational study.