Abstract: Objective:This study aimed to describe the inhibitory effects of different doses of radioactive seed 125I on the treatment of implanted tumor of moderately differentiated human gastric cancer cell line in nude mice. Methods: We developed20 nude mice models with implanted tumor of moderately differentiated human gastric cancer cell line. The mice were randomly divided into four groups with five mice in each group. Three groups were treated with radioactive seed 125I at different doses: 14 .8 MBq (group A);22 .3 MBq (group B); and 29 .6 MBq (group C). The control group (group D) was not subjected to radioactive seed 125I implantation. We constructed the growth curves of the implanted tumor and calculated the tumor inhibition rate in each group. The effects of the radioactive seed 125I implanted at different doses on tumor propagation were estimated by detecting the proliferating cell nuclear antigen (PCNA). The effects of induced cell apoptosis were determined by TdT-mediated dUTP nick-end labeling. Results:The volume and the weight of the implanted tumor in groups C and D were significantly reduced ( P<0.05 ). The tumor inhibition rates of groups A, B, and C were 10 .65 %,28 .65 %, and 37 .26 %, re-spectively. The PCNA expressions in groups B and C were also significantly reduced ( P<0.05 ). The inductive effects of apoptosis in groups B and C were significantly greater than those in the control group (P<0.05 ). Conclusion: Medium- and high-dose radioactive seed 125I could induce apoptosis and reduce PCNA expression. These findings may partially account for the inhibition of tumor growth. Thus, this method could be effectively used to treat human gastric cancer.