A simple and green method of depositing platinum-based bimetallic nanoparticles (Pt-Ru, Pt-Cu, Pt-Au, Pt-Pd, and Pt-Ni) on multiwalled carbon nanotubes (MWCNTs) in supercritical fluid carbon dioxide (sc-CO2) is described. CO2-soluble metal precursors, such as metal acetylacetonates or hexafluoroacetylacetonates, were used in the experiments. Suitable temperature and pressure conditions for synthesizing each kind of bimetallic nanoparticle are studied. Characterizations of these nanocomposites, performed by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD), all confirmed their presence. These MWCNT-supported bimetallic nanoparticles have average sizes varying from 2.8 to 9.3 nm. The application of these nanocomposites is demonstrated by using them as electrocatalysts for direct methanol fuel cell (DMFC). Their electrochemical activities are studied by using cyclic voltammetry (CV), and their efficiency to oxidize methanol to carbon dioxide is at least 60% better than the MWCNT-supported monometallic Pt nanoparticle catalyst reported previously.