The present study is carried out to investigate the influence of grain size on the level of radon radioactivity for calcium silicate (Ca2SiO4) ore rocks, which are used widely in many construction materials, particularly cement. The polymeric nuclear detector is used to determine radon activity concentration and assessment of the radiological impact of (Ca2SiO4) ore rocks. In addition to explaining the influence and quantification of grain size on radon radioactivity hence acts as a great radiological impact assessment and radiation protection to specify if it poses a risk to human health, moreover environmental safety. The investigated samples were analyzed by alpha trace detector (LR-115 NTD) to measure the radioactivity concentration of radon, which is produced in the continental crust by the natural decay series of uranium-238 and penetrates the pore spaces where it is imparted by diffusion and sent out into the atmosphere. The research results explained the correlation of grain size which influences the radon exhalation rate. The study indicated a normal radiation level (>1000 Bqm-3) regarding the international limitation and permissible levels recommended by IAEA, ICRP, and UNSCEAR.