The Pt nanoparticles were prepared by direct Pt sputtering onto trimethyl-n-propylammonium bis((trifluoromethyl)sulfonyl)amide (Me 3PrNTf2N) room-temperature ionic liquid (RTIL) without a stabilizing agent. The mean particle size (2.3 ±0 .1 run) was maintained substantially constant independently of the sputtering time if the sputtering time was less than 1800 sec. The obtained Pt nanoparticles were well-dispersed in the RTIL and could be immobilized on a single-walled carbon nanotube (SWCNT) by a simple process that was established in this research. The Pt-SWCNT composite material showed a remarkable catalytic activity to electrochemical oxygen reduction reaction more than a commercially available Pt nanoparticle-carbon composite catalyst. It suggests that the Pt-SWCNT composite material is one of the promising electrode catalysts for a PEM fuel cell system.