This in vitro study was made to evaluate the effect of gamma radiation on mechanical properties of tooth structure. A total number of 40 human premolar teeth were collected. The teeth were healthy and were freshly extracted and they were stored in distilled water for periods of less than one month the teeth were, free from any apparent caries,
macroscopic cracks, and abrasion and staining as assessed by visual examination.
The teeth were divided into 2 groups of 20 teeth each (A and B) The groups were: 1. Group (A) which served as a control.
2. Group (B) which subjected to 60 Gy of γ- radiation. The result showed the enamel microhardness increased after adose of 60 under the limitation of this study it was found that. The enamel microhardness increased at a dose of 60GY whereas the value of dentin microhardness decreases.