First-principles investigation on inelastic electron tunneling spectra of 1,6-hexanedithiol and 1,4-benzenedithiol molecular junctions with metal electrodes are performed.It shows that vibrational peaks in inelastic electron tunneling spectra are sensitive to electrode material and distance between sulfur atoms and electrodes.It hence provides unambiguous evidence that electrode material and metal-molecule bond length are crucial for inelastic electron transport through molecular junctions.The peak heights are mainly attributed to coupling energies between electrode and molecule.