Abstract: The ¹²⁵I is available for braehytherapy with its special biological and physical characters. At present, it is reported that good therapeutic effect of interstitial implantation of ¹²⁵I brachytherapy seeds used for malignant tumors has taken place widely. Interstitial implantation has a good look in the treatment of malignant tumors with the appearance of and the development of uhrasonograph and CT technology. At present, it has become one of many ways in the treatment of malignant tumors, but there are alse some problems to be resolved in the treatment of malignanl tumors.