Abstract: Objective To investigate the effect of Iodination-125 on vascular endothelial growth factor(VEGF) expression in breast cancer cells and the mechanism of Iodination-125 inhibiting the proliferation of tumor cells. Methods The animal models of breast cancer cell line MCF-7 were established by subcutaneously injecting the tumor cells into nude mice. The mice were divided into eight groups randomly and there were six nude mice in each group. The control groups contained C7, C14, C21 and C28 groups; the experimental groups contained T7, T14, T21 and T28 groups. No-load seeds(0Bq) were implanted into the control groups; Iodine-125 seeds（14.8MBq）were implanted into the experimental groups. The size of tumors was measured every two days and their volumes then were calculated. Anatomy analysis was done separately after 7, 14, 21 and 28 days of Iodine-125 seeds implantation; the mRNA and protein expression of VEGF were measured by RT-PCR and immunohistochemistry. Results After Iodine-125 seeds were implanted, the tumors in the experimental groups grew slowly, and the tumor inhibitory rate was 55.27%. After 28 days of Iodine-125 seeds implantation, the mRNA and protein expression levels of VEGF in tumors were significantly decreased in the experimental groups, compared with the control groups (P ＜0.01). Conclusion One of the important molecular biological mechanisms of Iodine-125 seeds inhibiting the proliferation of breast cancer cells is Iodine-125 seeds suppressing VEGF expression in tumor cells.