Radon generated in the earth's crust enters the pore spaces and transported by diffusion, then released into the surrounding atmosphere. Grain size is one of the important factors that effect on the radon exhalation rate from the soil. Radon emanates through the soil grains into air and diffuses to the atmosphere. The variation in radon concentration and exhalation rate from calcium silicate rock samples with different grain size were measured using Alpha Track Detector. The high value of radon exhalation rate was 43.05 ± 2.17 at gravel size and the lowest value was 8.98 ± 0.85 at mud size. The results indicate that the exhalation rate was increased with increase the grain size of the studied samples. The obtained values of radon exhalation rate for all the samples are found to be under the radon exhalation rate limit reported worldwide, and UNSCEAR. The present study is important to radiological impact assessment of requiring information on the exposure due to natural radiation to protect the environment.