Objective: To observe the effects of radioimmunotherapy with 90Y-OSMcAb on human osteosarcoma in nude mice osteosarcoma model. Methods:The monoclonal antibody labeled with carrier-free 90Y by singcyclic DTPA-chelation. The carrier-free 90Y was eluted from 90Y generator. The radioimmuno-complex was proven to combine with osteosarcoma cell. Osteosarcoma cells from human osteosarcoma cell line were implanted into the nude mice which could grow and reproduce in nude mice bodies. The therapy effects with 90Y-OSMcAb and 90Y-Cl3 on human osteosarcoma in nude mice model were observed. Results: (1) The 90Y activity was higher while the contumination of 90Sr was less than 0.0031%; (2) The efficiency of 90Y-labeled antibodies was over 90%，the activity of labeled antibody maintained 82.3%; (3) Tumor size of all 90Y-antibody-treated model was significantly less than that of all 90Y-Cl3-treated model, the average survival time of the mice of the 90Y-antibody-treated group was significantly longer than that of the 90Y-Cl3-treated group; (4) In the tissue sections extensive necrosis was observed in all 90Y-antibody-treated tumor, the living tumor cells were immunohistochemically stainable with PcNA. Staining with PcNA in all 90Y-antibody-treated tumor was negtive, but positive in all 90Y-Cl3-treated tumor. Conclusion: OSMcAb post-labeled targets osteosarcoma structures. 90Y-OSMcAbs can successfully treat osteosarcoma in nude mice.