The energy spectrum data of photon beams from a Cobalt-60 teletherapy unit is necessary not only for accurate radiation therapy but also for accurate dosimeter calibration. It is generally assumed that only 1.17 and 1.33 MeV gamma rays are present in the beams. However, this assumption is not completely valid because of energy degradation due to interaction of the primary gamma rays with the source material, source capsule, shielding treatment head and collimators. For this study the energy spectrum in a therapeutic beam from a Cobalt-60 teletherapy unit was calculated using a conscientious Monte Carlo simulation. The results were represented by the fluence rate per Bq in order to obtain relevant information for radiation therapy and dosimeter calibration. The proportion of unscattered primary photons to scattered photons was approximately 0.65: 0.35; the mean energy range of photons was 1.04 to 1.06 MeV. The Monte Carlo calculation of dose distributions was also made in order to confirm the validity of the energy spectra obtained in this study. The calculated PDD, using the energy spectrum data, agreed with the measured PDD in a wide range of depths. As a result, it is evident that the calculated photon energy spectra agree with the realistic energy spectra of therapeutic beams from a Cobalt-60 teletherapy unit.