When carbon powder is irradiated with a platinum-based plasma, the Pt+ ions collide with the carbon and become atoms, which then migrate and interact with other Pt atoms to form nanoparticles (NPs) with dimensions of 2 to 3 nm on the carbon surface. Figure 1 shows a scanning electron microscopy (SEM) image of Pt NPs on Ketjen black. Although the surface of the Ketjen black is extremely rough, the NPs are seen to be uniformly distributed. We compared the electrochemical properties of Pt/C catalysts prepared by APD with those of the commercially available catalyst TEC10E50E. Although the electrochemical surface area (ECSA) was almost the same, the specific activity for Pt/C(APD) was about 2 times higher than that for TEC10E50E, which led to an approximately 2 times higher mass activity. However, one remaining problem with Pt/C(APD) is that NPs are not deposited in some regions. Figure 2 shows an SEM image of Ketjen black containing regions where no Pt NPs are present. To address this issue, we made improvements to the stirring vessel, and optimized deposition parameters such as the plasma pulse interval and the powder surface conditions. We will describe the effects of these optimizations on the electrochemical properties of Pt/C(APD).