Fe3O4/Au composite particles with core/shell structure were prepared by reduction of Au3+ with hydroxylamine in the presence of an excess of Fe3O4 as seeds.The resultant colloids, with an average diameter of less than 100 nm, were obtained; the remaining non-reacted Fe3O4 seeds can be removed by treatment with diluted HCl solution.The Fe3O4/Au colloids exhibit a characteristic peak of UV-visible spectra, which largely depend on the size of the particle and the suspension medium.The localized surface plasmon resonance peaks red shift and broaden with increased nanoparticle diameter or increased solvent ionic strength.The optical property is very important in the establishment of means for the detection of biomolecules.