The perturbation of dose distribution and attenuation by back plate and metal frame of treatment couch in cobalt-60 teletherapy unit, Toshiba RCA-120-1. 4,000 Ci, was measured by various detectors and industrial film and prevented by using the styrofoam bed during irradiation as full rotation or four portal oblique technique. 1. Shielding the primary gamma ray beam by metal frame of treatment couch, the irradiated radiation dose was attenuated about 40-70%. 2. The tumor center dose due to shielding with back plate of treatment couch during irradiation as fur portal oblique combination was attenuated 20-50% according to patients thickness. 3. The dose attenuation by 1cm acryl plate was about 3% and the back skin surface get to the build up equilibrium state by secondary radiation from acryl plate, therefor the advantage of skin sparing effect from high energy photon irradiation could not expected. 4. Situated the treatment couch between the source and tumor center, about 10-20% attenuation of full rotation dose was occured. 5. Using the styrofoam-bad, air equivalent material, physical density, $ ho$=0.0015 and half value layer for cobalt-60 gamma ray, HVL=1.2$ imes$104cm, the attenuation of irradiated dose due to shielding of back plate and metal frame from treatment couch was completely prevented during rotation or four portal oblique irradiation.