The absolute optical oscillator strength density spectrum of carbon monoxide has been measured at an impact energy of 1.5keV and a mean scattering angle of 0° in an energy region of 7—21eV with typical FWHM 60meV. The absolute optical oscillator strengths corresponding to vibrational levels of A1Π,B1Σ+，C1Σ+ and E1Π are determined and compared with previous experimental and theoretical results. The reason why discrepancies exist among experimental results obtained by various methods has been explained and the limitation of dipole (e,e) method in determining optical oscillator strengths has been discussed.