Under certain acidic condition, thioglycolic-acid-modified gold nanoparticles (Au-NPs) can react with melamine, resulting in the aggregation of Au-NPs through electrostatic interactions between -COO(-) groups on the surface of Au-NPs and -NH3+ groups of melamine. The aggregation extent depends on the concentration of melamine, which can be validated by SEM imaging and UV-vis spectra. During the aggregation procedure, color change can be observed. Based on the color change, a simple, fast, cheap and visual colorimetric method of detecting melamine is developed, which can be effectively applied to detect melamine in milk powder and develop a new diagnostic kit of melamine.