Decorating graphene with high destiny and uniform size of noble metal nanoparticles is very important for a number of applications including sensing and electrocatalysis. Herein, a novel approach based on in situ diazonium chemistry was reported to design gold nanoparticles/graphene hybrid materials (Au-NPs/G). Graphene oxide was firstly electro-reduced on the glassy carbon electrode, and then this modified electrode was grafted by aminophenyl diazonium cations, generated in situ from p-phenylenediamine. Subsequently, gold nanoparticles (Au-NPs) were deposited on the p-phenylenediamine grafted electrode by a potentiostatic method. X-ray photoelectron spectroscopy, scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy were used to characterize the Au-NPs/G. The results demonstrated that the Au-NPs with high destiny and uniform size were dispersed on the electro-reduced graphene oxide. In addition, the obtained Au-NPs/G was utilized in electrocatalysis toward the oxidation of methanol showing excellent catalytic activity.