Dodecanethiol(DDT)passivated monodispersed Au nanoparticles (AuNPs) have been synthesized by using a combination of citrate reduction method and the Brust and schiffrin method（Two step-Two phase method）. The DDT-AuNPs can be easily adsorbed onto some substrates (silicon or glass) to form a thin film by using the dip coating method. The chemical composition and surface morphology of modified substrates were investigated by means of SEM. The optical properties were studied using UV-vis spectrum. The result indicated that the AuNPs can form a close-packed arrangement with a number of controlled plasma resonance effect and catalytic activity for CO oxidation at room temperature. It suggests that the film prepared by this method can be used extensively in chemistry biosensing and catalysis area.