Highly uniform Pt icosahedral nanocrystals with an edge length of 8.8 nm were synthesized in nonhydrolytic systems using the hot injection-assisted GRAILS (gas reducing agent in liquid solution) method. The results show the key factors for the shape control include fast nucleation, kinetically controlled growth, and protection from oxidation by air. The effect of oxygen molecules on the Pt morphology was experimentally confirmed based on the study of shape evolution of icosahedral crystals upon exposure to oxygen gas. The Pt icosahedral catalysts obtained had an area-specific activity of 0.83 mA/cm(2) Pt, four times that of 0.20 mA/cm(2) Pt for typical Pt/C catalysts, in an oxygen reduction reaction (ORR).