To design Pt-based materials with a hollow structure via a galvanic reaction would be one of the effective ways to prepare electro- catalysts with high activity. The galvanic reaction between Pt ions and metal template is usually conducted under limited conditions, which makes the preparation of Pt hollow nanoparticles laborious. Here, we introduce a one-step and one-pot synthetic approach for the preparation of carbon-supported PtNi alloy hollow nanoparticles with a narrow size distribution. Prepared PtNi alloys were characterized by a nonporous shell consisting of a Pt-enriched surface layer and an inner alloy layer of Pt and Ni. Due to its unique structural advantages, this material showed excellent electrocatalytic performance for oxygen reduction (3.3- and 7.8-fold enhanced mass and specific activities compared to those of a commercial carbon-supported Pt nanoparticle). A possible mechanism for the formation of PtNi hollow structure is suggested.