In this paper, we demonstrate a colorimetric visual detection of melamine in milk simply using citrate-stabilized Au nanoparticles (NPs). Due to strong interaction between melamine and Au NPs, all of the Au NPs sensors present a significant and sensitive color change in the presence of ppb-level melamine, which can be directly observed by the naked eye and monitored by UV-vis absorbance spectra. Au NPs size dependence on the visual detection sensitivity of melamine is firstly studied in detail. The visual detection sensitivity can be significantly improved from 15 ppb to 5 ppb when increasing the Au NPs size from 15 to 30 nm, and the optimized Au NPs size of the visual detecting sensor was determined to be 30 nm. The pH effect on the sensitivity results show that the optimal pH value was 3-4 and this approach shows excellent selectivity for melamine over the use of other molecules. More importantly, this size- and pH-optimized Au NPs sensor was successfully applied to visually detect a milk sample. As low as 10 ppb melamine in milk could be distinguished upon a color change of solution within five minutes. ; Department of Applied Biology and Chemical Technology ; Department of Applied Physics