Fabrication, optical property of nano coreshells and their potential applications in chemical and biological sensors have attracted much research interest. In this paper, we simulated the effect of size, structure, and dielectric properties as well embedding medium on the optical properties of gold or silver coated coreshell nanoparticle and the corresponding effects both of phase-retardation and of electron interface scattering as well as the nansize effect were discussed. Intriguing transition of the optical properties was simulated when a two-phase core-shell nanoparticle changes gradually into the single phase nanoparticle. We also obtained a good correlation of the simulation to the experimental results. Thus, the simulation has many implications and can guide further core-shell nanostructure fabrication with optimization of its optical properties.