This study was conducted to determine the optimal dose of gamma-ray for mutation breeding in sorghum (Sorghum bicolor M.). Gamma-rays irradiated to dry seeds with various doses (0 to 1000 Gy) at Korea Atomic Energy Research Institute. Lethal dosage (LD50) was approximately 256 Gy. Significant decreases in growth characteristics (plant height, tiller number and fresh weight) were observed by dose of increased over 300 Gy. Reduction doses (RD50) was approximately 363 Gy in plant height. We also conducted comet assay to observe nucleus DNA damage due to gamma irradiation. In comet assay, a clear difference was identified over 100 Gy treatments. With increasing doses of gamma-ray in the range of 100 to 400 Gy, the rate of head DNA was decreased significantly from 93.52% to 67.57%. The most of the Sorghum cells were severely damaged in the integrity of DNA by gamma-ray. These data provide valuable information when the optimal dose should be chosen for purpose of mutation breeding program of Sorghum.