Electro-oxidation is a way to utilize glycerol, a byproduct of biodiesel production, to produce fuels and feedstock chemicals for the chemical industry. A significant challenge is to get products with high selectivity, so it is desirable to understand the glycerol oxidation mechanisms in further detail. Using density functional theory calculations, we investigated possible glycerol oxidation intermediates on Pt(111) and Ag(111). We find that the different adsorption preferences of the intermediates on Pt (adsorption via carbon atoms) and Ag (adsorption via oxygen atoms) lead to different preferred reaction pathways, resulting in different products. The reaction pathways on both surfaces involve glyceraldehyde as a key intermediate; however, upon further oxidation, Pt(111) preferentially produces glyceric acid (CH₂OH-CHOH-COOH), while on Ag(111) C-C bonds are broken, which leads to the production of glycolaldehyde and formic acid (CH₂OH-CHO and HCOOH). These predictions agree well with the experimental outcome of the electro-oxidation of glycerol on Pt and Ag surfaces. Our study therefore provides useful insights for optimizing the selectivity of glycerol oxidation and improving the utilization of glycerol.