Monodisperse Pt nanoparticles supported on a graphitized carbon black (GC; 150 m² g⁻¹), which exhibits higher resistance to carbon corrosion than a conventional high-surface-area carbon black (CB; 800 m² g⁻¹), were prepared by the nanocapsule method. Three kinds of 50 wt%-Pt loaded catalysts (our nanocapsule Pt/GC, a commercial Pt/GC, and a commercial Pt/CB) were subjected to the durability test by a standard potential step protocol (E = 0.9 V ↔ 1.3 V vs. RHE, holding 30 s at each E, 1 min for one cycle) in N₂-saturated 0.1 M HClO₄ solution at 25 °C. The oxygen reduction reaction (ORR) activities at these catalysts were evaluated from the hydrodynamic voltammograms in O₂-saturated 0.1 M HClO₄ solution at 25 °C by the rotating ring-disk electrode technique. The kinetically-controlled mass activities (MA) for the ORR at these catalysts at E = 0.85 to 0.70 V vs. RHE were found to decrease in proportion to log [number of potential step cycles] from 100 to 5000 cycles. It was found that our nanpcapsule Pt/GC showed the highest durability; the time elapsed for the reduction of MA_0.8V to 700 A g⁻¹ (ca. 1/2 of the initial MA_0.8V) at our Pt/GC was 30 and 60 times longer than those for the commercial Pt/GC and Pt/CB, respectively. It was found that the most important factor leading to both high MA and high durability is highly dispersed state of Pt nanoparticles with uniform size over the whole surface of the corrosion-resistant GC support, to which our nanocapsule method has contributed greatly.