Au nanoclusters with the uniform size of 2 nm decorated on graphene (AuNCs-G) were successfully fabricated by a green photoreduction method., The composite was characterized by transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR). This one-pot method is cost-effective and convenient. The as-prepared hybrid is very "clean" as a result of the surfactants and stabilizers free formation process, allowing it to express excellent electrocatalytic ability, which was illustrated by using H2O2 as the target model. The formation mechanism of AuNCs-G was also interpreted by density functional theory (DFT), which was quite different from that of Pd nanoparticles decorated graphene. Lower concentration of AuC4- is favor of formation of smaller Au particles because graphene only has weak absorbility to Au atoms. This mechanism brings further direction to fabricate other noble metallic nanoclusters decorated graphene hybrids. (C) 2014 The Electrochemical Society.