Pt nanoparticles were deposited on mesoporous carbon support using two different Pt precursors, and the difference in mass activity toward oxygen reduction between the two was successfully examined through direct observation of Pt nanoparticles inside the mesopores. Existence of 1-nni-sized Pt particles on the wall of mesopores was observed, and catalytic activity was hi-her if the size of Pt particles inside the pores was smaller. The size and stability of Pt catalysts encapsulated in nanospace is key for catalytic activity.