A new method for the dynamic study of vehicle-bridge system has been presented here.The transferred spectra of the system's interaction due to roughness of road surface are studied by iterative method in frequency domain.The bridge and vehicle are treated as two subsystems and their equations of motion are solved alternately to derive the approximate 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 a result.