The core-shell gold nanoparticle film is fabricated by using nanolithography and self-assembly monolayer technology. The film exhibits unique optical properties and has strong surface enhanced Raman scattering (SERS) activity. The relationship between nanostructure and surface electrical field is studied by employing pyridine as the SERS probe. It is found that particle size and inter-particle space are important factors. The enhancement ratio is measured to be more than 10^4.