Objective:To study the effect of iodine-125 interstitial brachytherapy on liver cancer. Methods: Animal model of human liver cancer was established by injecting SMMC-7721 cells cultivated in vitro subcutaneously into the flank of BALB/c nude mice. Nude mice with tumor of 5 mm in diameter were randomly divided into 2 groups (n = 10). One iodine-125 seed of apparent activity 0. 8 mCi was implanted into the center of tumor in treatment group, whereas an inactive seed was implanted in control group. The other 20 nude mice with tumor reaching 10 mm in diameter were also treated as above. The size of tumor was determined weekly after implantation, and pathological examination and blood routine were taken on the 28th day. Results: Tumor growth was obviously inhibited in treatment group of tumor of 5 mm in diameter, and there was statistically significant difference in tumor volume between treatment and control groups (P<0. 01). Around iodine-125 seed, apparent necrosis of tumor was shown in treatment group, accompanied by karyopyknosis and reduced plasma in residual tumor cells microscopically. Tumor growth was not inhibited in either treatment or control group of tumor of 10 mm in diameter. There was no obvious adverse effect except for decreased white blood cells in treatment groups. Conclusion: There is certain effect of iodine-125 interstitial brachytherapy on liver cancer, which is associated with the size of tumor. Objective: To study the effect of iodine-125 interstitial brachytherapy on liver cancer. Methods: Animal model of human liver cancer was established by injecting SMMC-7721 cells cultivated in vitro subcutaneously into the flank of BALB / c nude mice. Nude mice with tumor of 5 mm in diameter were randomly divided into 2 groups (n = 10). One iodine-125 seed of the apparent activity 0. 8 mCi was implanted into the center of the tumor in treatment group, an an inactive seed was implanted in control group. The other 20 nude mice with tumor reaching 10 mm in diameter were also treated as above. The size of the tumor was determined weekly after implantation, and pathological examination and blood routine were taken on the 28th day. Results: Tumor growth was obviously inhibited in treatment group of tumor of 5 mm in diameter, and there was significant difference in tumor volume between treatment and control groups (P <0.01). Around iodine-125 seed, apparent necrosis of tumor was shown in t reatment group, accompanied by karyopyknosis and reduced plasma in residual tumor cells microscopically. Tumor growth was not inhibited in either treatment or control group of tumor of 10 mm in diameter. There was no obvious adverse effect except for decreased white blood cells in treatment groups. Conclusion: There is certain effect of iodine-125 interstitial brachytherapy on liver cancer, which is associated with the size of tumor.