In Britton-Robinson buffer of pH 2.5-3.5，famciclovir(FCV)could react with palladium(Ⅱ) to form 1∶1 anionic chelates.The chelates react further with chrome azurol S(CAS) to form the ion-association complexes,which could result in the great enhancement of resonance Rayleigh scattering(RRS) intensities and a new RRS spectra appearing.The maximum RRS wavelength locates at 367 nm.Under the optimum conditions，the intensities of RRS are proportional to concentration of FCV in the range of 0.02-2.4 mg/L，with a detection limit of 3.6 μg/L.The reaction conditions of the system and effect of coexisting substances were investigated.The softwares of Gaussview 3.07 and Gaussian 03W at a B3LYP/6-31G(base group)level based on the density functional theory(DFT) were used to calculate the charge distribution of FCV.The reaction mechanism and the reasons for RRS enhancement of the system were investigated.The method was simple,rapid and sensitive ,and was applied in the determination of FCV in capsule and urine samples with satisfactorily result.