An ionic-liquid-assisted method is developed for the preparation of noble metal nanoparticles supported on multi-walled carbon nanotubes (MWCNTs). The addition of a small amount of ionic liquids to the reaction solution leads to a production of well-crystallized metallic nanoparticles with tunable diameter and narrow size distribution, uniformly dispersed on MWCNTs. The obtained Pt nanoparticles supported on MWCNTs show superior catalytic performance. Both the catalytic activity and stability for the electrochemical oxidation of methanol is remarkably improved. The uniform size of Pt nanoparticles in the composites makes it possible to study the size dependence of the catalytic performance in the selective oxidation of glycerol. The turnover frequency (TOF) has the maximum value at the size of approximate to 4 nm and the selectivity to glyceraldehyde increases with the particle size.