We experimentally and theoretically study the density-dependent optical properties of 20 nm gold nanoparticle monolayers on silicon at 530 nm. Both results indicate a density-dependent resonance behavior. The density-dependent resonance suggests that the optical properties of the gold nanoparticle monolayers can be drastically modulated by changing the volume ratio V(r). It also implies, in this composite system, that the total surface plasmon resonance is maximized at a certain V(r).