When conventional gel electrophoresis was used, it was found that DNA of NIH3T3 cells cultured for different time after UVB irradiation had no DNA ladders, but thymocytes of Kunming mouse processed by the same method had DNA ladder. The results of field inversion electrophoresis showed that after UVB irradiation, the DNA of NIH3T3 cleaved into high molecular weight fragments at first, and then into low molecular weight fragments, still without the appearance of DNA ladders. It is suggested that DNA cleavage of apoptotic cell is not always initiate at internucleosome.