The concentration dependence of surface enhanced Raman scattering (SERS) of adsorbed polybenzimidazole on surfaces of silver has been investigated. It has been found that the spectral qualities are inversely proportional to the concentration of solutions applied. Excellent SERS spectral enhancement could be achieved for the sample applied from wt-0.03% solution. This phenomenon was termed as “dilution effect” on SERS spectrum of polymers. The observed phenomena are in accordance with Qian's proposal for the existence for coil shrinkage in dilute solution.