In this work, we have successfully synthesized free-standing ultrathin Pt-Cu alloy nanosheets of 4-6 atom thickness with tunable lateral size from 10 to 50 nm. The nanosheets with diameters >20 nm can be converted into nanocones in a controllable way. These nanosheets and nanocones exhibit excellent electrocatalytic activities for the oxidation of ethanol in comparison to commercial Pt black and Pt/C catalysts.