The concentration dependence of surface enhanced Raman scattering (SERS) of adsorbed 4-aminophenyl disulfide on surfaces of silver and gold has been investigated. It has been found that the spectral qualities are inversely proportional to the concentration of solutions applied. Excellent SERS spectral enhancement and perpendicular orientation could be achieved for the sample applied from 4 × 10−6Msolution rather than that from 0.001Msolution. This phenomenon was termed as “dilution effect” on SEES spectrum.