Platinum nanoparticles were fabricated on multiwalled carbon nanotubes (MWCNTs) by using supercritical fluid deposition. In this technique, deposition was carried out in a supercritical carbon dioxide fluid via hydrogen reduction of a dissolved platinum complex. The deposition temperature and deposition-time dependences on the particle size and density were investigated. The obtained metallic nanoparticles/MWCNT nanocomposites were characterized by using transmission electron microscopy and scanning transmission electron microscopy. Crystalline pure platinum nanoparticles were clearly observed on the surfaces of MWCNTs. Most of the particles observed were smaller than 5 nm in diameter. The density of the particles increased with temperature between 393 and 423 K and decreased above 423 K. The nucleation of the nanoparticles took place within a deposition time of 60 min, and then agglomeration and coarsening occurred, resulting in an increase of the particle size. It was found that the surface topography of the carbon support greatly influences the platinum nucleation density. Indeed, when the MWCNTs were treated with hydrogen plasma prior to platinum deposition, the density of the nanoparticles markedly increased. The impact of the hydrogen plasma treatment on the particle-size dependences was also investigated.