Inter-tissue implantation of radioactive particles for the treatment of malignant tumors belongs to the category of internal radiotherapy, which is the implantation of radioactive microparticles into tumor tissues through CT-guided positioning technology and treatment planning system (TPS) to achieve the purpose of treating malignant tumors. At present, the most used radioactive particle at home and abroad is 6711 type 125I, which has an activity of 0.8 mCi, a half-life of 60.2 d, an energy of 27.4 X-rays and 35.5 keV γ-rays. The specific structure is that the radionuclide with the effect of killing tumor cells is placed in a sealed metal titanium shell, which is sealed and made into a small short rod of less than half a centimeter in length like a 0.8mm pencil lead used by elementary school students, so it is called “particle”. The operation process is to input the tumor size shown by CT and MR films into a TPS planning system, and then restore it into a three-dimensional image with the actual size of the tumor, and then precisely distribute the particles evenly in the tumor tissue with a spacing of 1cm in length and 1cm in width at each interval. In this treatment plan, doctors use CT to guide and place the radioactive particles directly into the tumor with a set of special instruments, just like “mine-laying”. The radiation emitted by these particles inside the tumor can rapidly kill the tumor cells, and the chance of losing the tumor treatment volume is greatly reduced because there is no influence of body position and breathing movement during the irradiation process. This minimizes the impact of radiation on normal tissues, organs and systems of the body, while the tumor target area receives high doses of radiation with minimal associated acute radiation damage and long term effects. The damage to patients and health care workers is also very mild, and the procedure is easy and efficient, without the need for secondary surgery to remove the radiation source, which makes it very popular in clinical applications. The first case of radioactive particle interstitial therapy in China was reported in 2001, and it has been developed for more than 10 years since then. It has satisfactory clinical efficacy in the treatment of many types of tumors, especially in the treatment of liver tumors, lung tumors, retroperitoneal tumors, pelvic tumors, breast tumors, bone metastases, malignant lymph nodes, etc. The clinical cases reported in the literature have shown encouraging tumor control rates. This treatment item belongs to the third category of medical technology strictly regulated by the Ministry of Health, and our department has obtained the qualification to carry out it and has achieved good results in some patients. This lecture is about the clinical status of radioactive particles, radiation characteristics, specific clinical procedures, typical clinical cases and the key points of intraoperative and postoperative protection management.