Ag and Au nanoparticles were self-assembled on a silicon substrate simultaneously. The density of Ag and Au nanoparticles was controlled by changing the volume ratio of Au to Ag colloids. The substrate was characterized by UV-Vis diffuse reflectance and SEM, and the results indicated that Ag and Au nanoparticles were dispersed on the substrate with mono/submono layers. Using SCN- as a probing molecule, surface enhanced Raman spectroscopic (SERS) effect was investigated on pure Au, pure Ag and Au-Ag mixed nanoparticle surfaces. After a series of calibration, the results revealed that the SERS spectral features of SCN- from the mixture system were similar to those from the pure Ag nanoparticles, indicating the coupling effect between the Ag and Au nanoparticles, which results in the property of Au inclining to Ag.