Inelastic electron-tunneling spectroscopy (IETS) has been used to investigate the vibrational spectrum in Al/Al₂O₃/CO/AI tunneling junctions with various Co thicknesses (d _Co). A zero-bias anomaly was observed in the conductance curve of the junction with d_Co of 2Å, and decreased with increasing d_Co. The IET spectra of these junctions showed strong negative peaks at 4 mV, corresponding to the zero-bias anomaly, while phonon spectra were observed for the junction with d_Co ≥ 10Å. The peak position was different from that of Al/Al₂O₃/Al. After annealing of the junction with d_Co of 2Å at 250°C for one hour, the zero-bias anomaly decreased and the phonon spectrum appeared.