Au Au 2S nanoshell consists of a nanometer scale Au 2S dielectric core coated with a thin Au shell. The new electromagnetic resonant cavity properties of the gold nanoshell are discussed. The gold nanoshell is regarded as a nanometer electromagnetic resonant cavity, and the cavity resonance displays tunability controlled by the radii of the dielectric core. The main mode of nanocavity resonant absorption is obtained, it shows red shift during the reaction of the gold nanoshells. The parameters of gold nanoshell cavity resonance such as resonant wavelength, energy and linewidth are discussed.