To study whether prolonged or repeated low-dose ionizing radiation could induce female breast cancer, we analyzed the incidence and risk factors of breast cancer among female medical diagnostic X-ray workers in China by cohort study and case-control study nested in the cohort. The risk of breast cancer enhanced significantly than the control group. It occurred in those who engaged in X-ray work before 1960, those who have been worked more than 25 years and those who are exposed before age 30, however, the age of onset cancer did not advance. The significant risk factors are accumulative radiation dose of the breast, obesity and family history of breast cancer. In addition, interaction could exists between obesity, non-lactation history and occupational X-ray exposure.