Cubic Pt nanoparticles were prepared by NaBH4 reduction of K2PtCl6 in an aqueous medium. Under the effect of phase-transfer inducer HCl, Pt nanoparticles were transferred to a toluene solution containing dodecanethiol, forming a toluene organosol of cubic Pt nanoparticles protected by dodecanethiol. A monolayer of cubic Pt nanoparticles is obtained on carbon film by self-assembly using this organosol. The stability of the organosol is checked; after being aged for 10 months, the same monolayer of cubic Pt nanoparticles can be obtained using the organosol. The organosol, Pt nanoparticle self-assembled monolayer, and phase transfer were characterized by UV-vis spectra and transmission electron microscopy.