Four types of cells, melanoma B16, cervical squamous carcinima HeLa, Chinese hamster V79 and hepatoma SMMC-7721, were irradiated by 125. 5 keV/j^m carbon ions. Celullar sensitivities to irradiation indicated by D_50, DNA double strand break (DSB) and distribution of DSB fragments expressed by molecular weight are studied. The results show that there is not a consistent relationship between cellular sensitivity and DNA DSB induction, a possible molecular mechanism of radiosensitivity which is defined as "a complementary mechanism of DNA sequence sensitive sites in cooperation with DNA DSB induction" is proposed. It gives a possible explanation of different sensitivity in four types of cell lines.