In this study, we tried to define the frequency of micronuclei (MN) and morphological damages in peripheral blood samples of cancer patients exposed to radiotherapy (RT) for lung tumors. For this purpose, we used in vivo cumulative dose-effect relationship, and correlated these data with statistical parameters. Cytological analyses were performed on white blood cells (WBC) of patients exposed to γ–radiation. MN assay was used as a biomarker of radiation damage. Besides, the effect of different factors like smoking status and age was investigated by MN assay. Scoring of MN was performed on mononucleated lymphocytes. Peripheral blood smears were prepared for morphological characterization and the number of WBC. The results indicated that, morphological damages and MN frequency in WBC were significantly higher in cancer patients exposed to γ–radiation than in the controls, and difference was statistically significant (P<0.01). The morphological changes such as vacuolization, membrane defects and increase in cytoplasmic granulation were detected in WBC. Moreover, an increase in the frequency of MN and a decrease in the numbers of WBC were observed depending on radiation dose. In conclusion, it was showed that morphological damages and MN frequency are very sensitive and useful biomarkers for the investigation of the effects of RT.