Abstract: A new method for the dynamic study of vehicle-bridge system has been pr-esented here. The transfer spectra of the system's interaction due to road surf -ace roughness have been studied by iterative method in frequency domain.The bridge cvnd vchicle are treated as two subsystems and their equations of motion are solved alternately to derive the approxmate response spectra.In this paper the lowest frequency of the system and its dynamic stability are investigated by spectral analysis.the advantageous vehicle speeds have been obtained As the result.