The shielding characteristics of natural bentonite can be enhanced based on calcination and ball-milling processes for protection against gamma radiation. The calcination process increases the content of the oxide, which enhances the mass attenuation coefficient; however, the elimination of water and organic matter from bentonite clay structures increases the particle size, where large particle size has a negative effect on this mass attenuation coefficient. Therefore, the calcinated bentonite has been ball-milled to reduce the particle size and improve the attenuation properties of natural bentonite. Furthermore, the calcination process occurs at 700°C for 2 h because dehydration is completed above 500°C while dehydroxylation is observed at 700°C. Therefore, the shielding parameters have been determined for calcinated, ball-milled, pressed bentonite clay samples according to different gamma-ray energies (662, 1173, and 1332 keV), where the experimental setup is based on narrow beam transmission techniques with two sources (137Cs and 60Co). In addition, the particle size of bentonite clay has been characterized using X-ray diffraction patterns depending on two different methods: dynamic light scattering and Williamson-Hall size analyses. This study shows that the calcinated, ball-milled bentonite pressed at 150 bar has the highest linear and mass attenuation coefficients of μ = 0.13 cm−1 and μm = 0.082 cm2/gm, respectively. Moreover, the experimental and theoretical investigation of the mass attenuation coefficient is in good agreement.