Ag-coated Au composite nanoparticles are prepared via solution-phase reduction method and clearly observed by TEM. The optical absorption spectra show that the as-prepared particles have two distinct plasma a absorption bands. One absorption peak centers near the plasma a absorption band of pure Ag nanoparticles, another absorption peak intervenes between the plasma a absorption bands of pure Ag and pure Au nanoparticles and undergoes blue-shift. According to the experimental analysis and numerical calculation based on Mie scattering theory, it is reasonable to attribute the blue-shift to the thickness increasing of Ag-shells.