The microdosimetric-kinetic (MK) model is one of the models that can describe the fraction of cells surviving after exposure to ionizing radiation. In the MK model, there are specific parameters, k and y(D), where k is an inherent parameter to represent the number of potentially lethal lesions (PLLs) and y(D) indicates the dose-mean lineal energy in keV/mu m. Assuming the PLLs to be DNA double-strand breaks (DSBs), the rate equations are derived for evaluating the DSB number in the cell nucleus. In this study, we estimated the ratio of DSBs for two types of photon irradiation (6 MV and 200 kVp X-rays) in Chinese hamster ovary (CHO-K1) cells and human non-small cell lung cancer (H1299) cells by observing the surviving fraction. The estimated ratio was then compared with the ratio of gamma-H2AX foci using immunofluorescent staining. For making a comparison of the number of DSBs among a variety of radiation energy cases, we next utilized the survival data in the literature for both cells exposed to other photon types, such as Co-60 gamma-rays, Cs-137 gamma-rays and 100 kVp X-rays. The ratio of DSBs based on the MK model with conventional data was consistent with the ratio of gamma-H2AX foci numbers, confirming that the gamma-H2AX focus is indicative of DSBs. It was also shown that the larger y(D) is, the larger the DSB number is. These results suggest that k and y(D) represent the characteristics of the surviving fraction and the biological effects for photon irradiation.