Two models of p-hydroxybenzoic acid (PHBA) adsorbed on the surfaces of silver nanoparticles were established, each of them corresponding to an experimental configuration. The first model is PHBA adsorbed on one Ag atom through the carboxyl group, and the second one is PHBA adsorbed on two Ag atoms through the carboxyl and hydroxyl groups. The Raman spectra of these two models using DFT-B3PW`91 with lanl2dz were calculated; it was found that the calculated Raman frequencies were in good agreement with experimental values. So one can conclude that the simplified models are probably reasonable to describe some surface-enhanced Raman experiments.