Abstract: Objective To study the inhibitory effect of radioactive seeds ¹²⁵I through interstitial implantation on neoplasm invasiveness, and analyze possible mechanisms. Methods The BGC-823 cells were subcutaneously injected into the right anterior armpit of BALB/C nude mice in order to establish the model of metastatic tumor. The mice of metastatic tumor were randomly divided into control group and experimental group, and mice from the experimental group were treated by interstitial implantation of ¹²⁵I for 30, 60 and 90 days, respectively. The tumor inhibitory effect was detected by measuring tumor weigh, size and lung metastasis nodules. The levels of telomerase protein in metastatic tumor, VEGF and bFGF in serum were measured by ELISA, and the expression of p-Erk1/2 in tumor was analyzed by Western blot. Results Compared with the control group, tumor weight, size, lung metastasis nodules, and the levels of telomerase protein in metastatic tumor, VEGF and bFGF in serum were significantly decreased due to 125I interstitially implanted. Meanwhile, the expression of p-Erk1/2 in tumor was down-regulated (P<0.05 or P<0.01), especially for 90-day treatment. Conclusion It could be the mechanism that Erk signal pathway in volved in neoplasm invasiveness is inhibited by radioactive seeds ¹²⁵I through interstitial implantation in xenograft nude mice of human gastric carcinoma.