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We report a new catalytic system by partially covering the uniform Pt nanocrystals on a carbon support with an ultrathin film derived from polyacrylonitrile (PAN). The use of Pt nanocrystals uniform in both size and shape effectively suppresses Ostwald repining, while partially covering them with a PAN‐derived film prevents migration, aggregation, and detachment from the support. In addition, the pyridinic N atoms on the edges of the thermally‐treated PAN film can also weaken the O=O bond, accelerating the reduction of oxygen. Upon optimization, the new catalyst exhibits a mass activity of 0.51 mA ⋅ μg⁻¹_Pttoward oxygen reduction, substantially enhanced relative to the same catalyst without PAN (0.22 mA ⋅ μg⁻¹_Pt) and a commercial Pt/C (0.41 mA ⋅ μg⁻¹_Pt). The mass activity is essentially retained after 10,000 cycles of accelerated durability test between 0.6 V and 1.1 V in oxygen‐saturated HClO₄. Even after aging in H₃PO₄at 220 °C for one week, the electrochemical surface area of the catalyst is still maintained. This catalytic system holds great promise for use in various types of fuel cells with a long lifetime.