This paper is to achieve a gamma-ray source with the lowest rate of buildup factor, which is of great importance in medical, industrial and agricultural sciences. The flux buildup factor of gamma rays is calculated by the MCNP code for point, linear, surface and volume sources with shield layers of lead, iron and aluminum. The results show that for the high Z shielding material, the flux buildup factor of  coaxial cylindrical sources is the lowest (۱.۶–۲.۳) of all sources, while for low Z shielding materials, the coaxial disk surface sources have smaller buildup factor (۱.۴۵–۱.۶).