Abstract: Objective To investigat the mode of cell death after irradiation in one human lung adenocarcinoma cell line and a human normal lung fibroblast cell line. Methods The morphology of dead cells and dou-ble strands break in apoptotic cells were observed by HE stain and comet assay.The variety of cell cycle and emerging time of apoptosis after irradiation were examined by flow cytometry analysis. Results Apoptotic cells emerged in the lung adenocarcinoma cells after irradiation and appeared in G1 arrest subsequent to overcoming G2 arrest after irradiation.Only was necrosis observed in the fibroblast cells. Conclusion Apoptosis is one of the ultimate forms of cell death via mitotic failure caused by irradiation in lung adenocarcinoma cells.The advent of mitotic apoptosis depended on the radiation doses and incubation time. The fact that mitotic mecrosis might be the major cause of fibroblast death demonstrated that mitotic death modes were complex and various in different lines.