Colloidal PtCu alloy nanoclusters (NCs) stabilized with acetate ions and ethylene glycol, with small particle size (similar to 2 nm) and tunable Pt/Cu ratio, are successfully synthesized by a modified alkaline-ethylene glycol method. As tractable building blocks, the prepared PtCu NCs are immobilized on a melem-modified carbon support (MMC) to assemble electrocatalysts (PtCu/MMC) for oxygen reduction reaction (ORR). The influence of Pt/Cu ratio in the alloy NCs of PtCu/MMC on the catalytic activity for ORR is investigated. Pt75Cu25/MMC exhibits the highest electrochemical mass activity (1.59 A mg(pt)(-1)) and specific activity (3.98 mA cm(pt)(-2)) at 0.9 V of supported NCs ever reported. After an aging test of 10,000 potential cycles, the loss of mass activity over Pt75Cu25/MMC is only 9%, which is the best durability compared to previously reported PtCu NCs catalysts measured at the similar conditions. Colloidal PtCu alloy NCs and the immobilization method reported in this work will find wide applications in the development of bimetallic catalysts.