Few ellipsometric studies have been conducted on Au self-assembled monolayers (SAM), with large discrepancies obtained for refractive index values. Because the Au NPs layer is a kind of composite layer, Au NPs and void, the effective media approximate (EMA) model was employed to investigate the optical properties of Au NPs. The reflective coefficient (approximately 1.2) was smaller than the extinction coefficient (approximately 2.5), which is different from the previous reports and corresponds to the Drude model. The absorption peak of surface plasmon resonance for Au NPs on the glass substrate was shifted from 580 nm to 480 nm.