Forming core-shell and alloy structures offers generally two ways to design efficient Pt-based catalysts for oxygen reduction reaction (ORR). Here, we combined these two strategies and invented a versatile aqueous route to synthesize octahedral [email protected] core-shell nanocrystals. The Pt/Ni atomic ratios in the resultant shells can be varied from 0.6 to 1.8, simply by changing the amounts of Pt and Ni precursors, with the other conditions unchanged. Experimental studies showed that the mass activities of as-prepared catalysts were 5 times higher than that of the commercial Pt/C. We believe that the ultrathin PtNi shells enclosed by {111} facets made it possible to reduce the Pt content while retaining the catalytic activity toward ORR. This strategy may be extended to the preparation of other multimetallic nanocrystals with shaped and ultrathin alloy shells, which is conducive to design highly active catalysts.