THE objective of this study is focused on teletherapy unit to investigate the effect of Polymethyl Methacrylate blocking tray on the surface and build-up doses within 0.5 cm thickness for 60Co γ-ray beams with various fields. The percentage depth dose (PDD) in build-up region for blocking tray out and in the path of the beam was measured by a Markus chamber, which was irradiated by a 60Co beam. Irradiations were performed in virtual water phantom at various depths and fields.The polarity effect and the over-response correction factor were applied for Markus chamber.The PDD of 4.5×4.5 and 35×35 cm2 fields found to be 16.70%±0.14% and 67.12%±0.25% at surface for blocking tray out, and 16.86%±0.29% and 72.47%± 0.59% for blocking tray in place, respectively. Therefore, blocking tray in place was higher than the dose for tray out, but the increase was dominant for large fields. This behavior is seen in all depths in build-up region with different ratios. The maximum variations of PDD for 5×5 cm2,10×10 cm2, 25×25 cm2 and 35×35 cm2 was found at surface and were in the order of 0.15%, 0.49%, 3.23% and 5.35%, respectively, this increased with increasing field and decreased with increasing depth .The dose in the build-up region increases with increased field for blocking tray out and in place. For blocking tray in place, the dose in the build-up region was higher than that for tray out. This effect can be due to the blocking tray eliminating secondary electrons and at same time generating new electrons as photon interacts with the tray material.