Different sized Pt nanoparticles supported on carbon nanotubes were prepared and then applied in selective oxidation of glycerol in a base-free condition to understand the size effects. It is shown that the turnover frequency of glycerol increases with Pt particle size to a maximum at the mean size of 2.5 nm followed by a decline with a further increase in size, which may be due to strongly adsorbed intermediates blocking Pt active sites for smaller sized Pt catalyst. Moreover, compared with dihydroxyacetone, glyceraldehyde and subsequent glyceric acid (GLYA) are dominating products. In particular, smaller sized Pt catalyst favors the formation of GLYA, which could be related to the stronger oxidation. Unexpectedly, the selectivity of C3 products is insensitive to both Pt particle size and reaction time within 9 h.