DNA polymerase β (pol β) is the base excision repair (BER) polymerase which removes oxidative lesion bases caused by chemotherapeutic agents. Adriamycin (ADM) has been found to kill tumor cells through producing oxidative free radicals, thereby leading to DNA/chromosomal damage as well as gene mutation. Deregulation of the expression of pol β may be important in the predisposition of cell to adriamycin. Here, we investigated if knock-out of pol β gene could increase cellular sensitivity to adriamyicn and try to reveal the potential mechanism. In this study, we used murine embryo fibroblasts wild-type (pol β +/+) or homozygous null for pol β (pol β -/-), which plays a central role in BER. Cellular sensitivity was examined using MTT assay. DNA/chromosomal damage and gene mutation were assessed by comet assay, micronucleus assay and hprt gene mutation assay respectively. Cellular ROS level was measured by a sensitive fluorescent probe method. The results showed that, compared with pol β +/+ cells, pol β -/- cells were sensitive to adriamycin. Furthermore, ROS increased much more in pol β -/- cells, leading more severe DNA/chromosomal damages as well as higher hprt gene mutation frequency. These observations indicated knock-out of pol β would let ADM-induced DNA oxidative damage can not be repaired resulting in the hypersensitivity to adriamycin, which suggested a role for this polymerase in providing to lerance to adriamycin-induced damage. Our study predicts that pol β could be considered as a potential drug targets to sensitise tumors to chemotherapy.