In order to study the effect of drug resistance on the radiosensitivity of human ovarian cancer lines exposed to ,60,Co γ-rays at low absorbed doses, the human ovarian cancer cell line A2780 and its paclitaxel (PTX)-resistant subline A2780/Taxol were used as a model of the cells. MTT assay was used to determine the sensitivity of the cells to paclitaxel and radiation. Radioresistance of cells was calculated by colony formation test. The real time PCR was used to determine the variation in the expression of mRNA of MDR1 in the model cells before and after radiation. It has been found that the resistance index (RI) of A2780/Taxol cells is up to 43.96, while the relative survival rate of A2780 cells is more significant than that of 2780/Taxol cells with increment of radiation absorbed dose (,p,<,0.05). The results indicate that the PTX-resistant cell line A2780/Taxol has significant radio-resistance. Drug and irradiation can induce over expression of MDR1, which may be one of the main reasons of crossed resistance to chemotherapy and radiotherapy in human ovarian cancer.