Ionizing radiation (IR) can induce multiple cellular responses which determine the fate of cells, including chromosomal aberrations, genetic mutations, cell cycle arrest, apoptosis, and cell differentiation. IR also enhances the production of reactive oxygen species (ROS) in a variety of cells, causing DNA damage and activating several proteins involved in the DNA repair system. The balance between DNA damage and repair is a key determinant of intrinsic radiation sensitivity. In this study, we investigated the survival rate in different types of cell lines (HUVEC, Bud-8, CCD-18Lu and Jurkat) after irradiation at various doses of $gamma$-radiation, and compared the radiation-induced ROS generation with cell viability. We also measured micronuclei frequency in the irradiated cells in order to verify the cytogenetic analysis by radiation. In addition, we analyzed the cellular level of several proteins related to the cell cycle in order to characterize the correlation between the intrinsic radiosensitivity and cell cycle regulation. We found that the intrinsic viability of irradiated cells was correlated with the radiation-induced ROS production as well as the function of the organ of origination. We also observed increased cell cycle protein expression in radiation-sensitive cells compared to radiation-resistant cells. These results suggest that intrinsic cellular sensitivity to irradiation is dependent on ROS generation and cell cycle regulation.