Ionizing radiation is a potent inducer of DNA damage and comet assay provides a very sensitive method for detecting strand breaks and measuring repair kinetics in single cells. The DNA repair capability influence cell sensitivity to ionizing radiation and predicting radio sensitivity of a cell population is very important for radiation therapy. In the present study the effect of gamma radiation on inducing DNA damage in two cell lines, normal human keratinocytes (HaCaT) and human tumor adenocarcinoma (HT 29) has been analyzed. Radiation treatment has been carried out using gamma radiation from a 60co source and the induction and repair of DNA strand breaks were quantified using alkaline comet assay. Repair studies were performed by incubating the sample at 37°c for different recovery times with the aim of elucidating repair kinetics. The relation between initial DNA strand breaks and the rejoining kinetics of the strand breaks has been studied in the present investigation. The average repair half time for both the cell lines has been found out. Heterogeneity in DNA damage within the cell population was observed as a function of radiation dose and repair time. It is observed that a comparatively radio resistant tumor cell line HT 29 has a shorter repair half time compared to that of normal cell HaCaT. Both the cell lines showed a dose-dependent repair activity and it indicates that the repair rate is proportional to the induced damage