We have studied the ultrafast dynamics of coherent phonons in sapphire crystals irradiated with 60Co γ-rays for three different doses by femtosecond time-resolved coherent anti-Stokes Raman scattering (fs-CARS) technique at room temperature. The obtained fs-CARS signals exhibit well-defined quantum beats, which are ascribed to the interference of the 645 and 750 cm-1 phonon modes. The dephasing times of the two modes both decrease with increasing irradiation dose, which is due to the scattering of coherent phonons by the defects introduced by γ-ray irradiation.