To determine the lethal dose (LD) and growth-reducing dose (GR), the exposures were from gamma activity rates such as low activity rate by multipurpose panoramic 70 Ci and high activity rate by Gamma Cell 3 kCi. The study material was sourced by Cobalt-60 (⁶⁰Co) with variant doses, i.e., 10Gy, 20Gy, 30Gy, 40Gy, 50Gy, and 60Gy for each gamma activity rate. The study was performed at the Center for Isotope and Radiation Application, National Nuclear Energy Agency of Indonesia (CIRA-NNEA). Data were analyzed using non-parametric tests and analysis of variance. The lethal dose (LD₅₀) and growth reduction (GR₅₀) were identified based on regression analysis. The analysis of variance revealed that highly significant differences among irradiation treatments in number of leaves, survival rate, and plant height. A linear regression model was developed to determine the mean LD₅₀ and GR₅₀ of Super Napier grass. The highest variability of mutants was observed in gamma-ray irradiated mutants with lethal doses (LD₅₀). LD₅₀, 35.82Gy-55.71Gy, at a low activity rate (Multipurpose panoramic irradiator 70 Ci) is higher than the high activity rate (Gamma Cell 3 kCi), 28.98-28.99. In general, the survival rate of Super Napier grass decreased along with increasing irradiation dose. The generated LD₅₀ and GR₅₀ were identified as optimal dosages for the radiosensitivity of Super Napier grass. This study becomes preliminary mutagenesis breeding to generate genetic diversity of grass, specifically in Super Napier grass.