The Monte Carlo transport code MCNP was used to simulate the photon beam from a Theratronics 780-C cobalt therapy unit and to calculate some dose-dependent parameters as functions of field size. The simulation process has included the source capsule, collimators (fixed and adjustable), lead in the unit head, and the field sizes as ranged from 5 × 5 to 35 × 35 cm2. Calculations have been carried out in a water phantom at a fixed source-surface distance of 80 cm. Detailed simulation of the major components of the therapy unit made it possible to calculate the effects of each unit component on the photon spectrum at the phantom surface. Percentage depth dose and peak scatter factor were evaluated for various field sizes. And tissue-air ratios were also determined for a field size of 10 ×10 cm2, as a function of depth down to 30 cm. To test the accuracy of the calculated results, they were compared with the published data of the British Journal of Radiology (BJR) suppl. 25 and good agreement between measurements and calculations has been obtained. Deviations typically were found to be of the order of 1%.