OBJECTIVE To investigate an initial approach of radiotherapy,which produces cellular radiation injury by high dose in one fraction.METHODS Human pancreatic carcinoma cell line MIA PaCa-2,was cultivated and divided into 5 groups: 0, 2, 5, 10, 17 Gy.Cultivated cells were irradiated by 6MV-X ray in one fraction.Analysis were done as follows: comet assay, which assessed the level of DNA damage in the treated cells right after the cell was irradiated, flow cytometry, which was performed at 0, 6, 12, 24, 36h after the cell line treated to asses changes of its cell cycle, DNA ladder, which quantitatively assessed the degree of DNA injury after 6 and 12 h, and histological examination, which analyzed cellular morphology after 24 h.RESULTS (1) After X-ray irradiated, the morphological change of human pancreatic carcinoma cell line (MIA PaCa-2) was mainly swelling. (2) When the dose of radiation was lower than 10 Gy,increasing the dose could greatly improve cell necrosis, apoptosis and blockage of cell cycle in G2/M phase, which was consistent with the theory of radiation biology. (3) When radiation dose was more than the 10 Gy, the peak of apoptotic necrosis appeared strong and early. (4) The degree of DNA injury was also related to the dose of radiation therapy and most obvious in the 10 Gy group and not so obvious in the 17 Gy group. (5) When dose was less than 10 Gy, DNA ladder was a single electrophoretic band; in the 10 Gy group, the electrophoresis showed a multiple ladder band;when dose was more than 10 Gy, a vague and irregular band appeared on the electrophoresis.CONCLUSION Oncotic necrosis may be the main cell death style when dose per fraction is high, which differs from conventional dose fraction radiation therapy.