Research on ionizing radiation (IR)-induced clonogenic cell death is important for understanding the effect of IR on malignant tumors and normal tissues. Here, we describe a quick and cost-effective one-step assay for simultaneously assessing the major modes of clonogenic cell death induced by IR, i.e., apoptosis, mitotic catastrophe, and cellular senescence. In this method, cells grown on a cover slip are irradiated with X-rays and stained with 4',6-diamidino-2-phenylindole dihydrochloride (DAPI). Using fluorescence microscopy, apoptosis, mitotic catastrophe, and cellular senescence are identified based on the characteristic morphologies of the DAPI-stained nuclei. Apoptosis is determined by the presence of apoptotic bodies (i.e., condensed and fragmented nuclei). Mitotic catastrophe is determined by the presence of nuclei that exhibit two or more distinct lobes and micronuclei. Cellular senescence is determined by the presence of senescence-associated heterochromatic foci (i.e., nuclear DNA containing 30-50 bright, dense foci). This approach allows the experimenter to easily screen for clonogenic cell death modes using various cell lines, treatment settings, and/or time points, with the goal of elucidating the mechanisms of cell death in the target cells and conditions of interest.