Objective To investigate the effect of the semi-joint prosthesis replacement in treating malignant tumors around the children’s knees. Methods Five children (aged 8-12 years) with malignant tumors around the proximal end ofthe tibia underwent the semi-joint prostheses replacement from March 2000 to June 2005. All the children had been diagnosed with osteosarcoma, which was graded as ⅡB by the Enneking staging system. The pathologic changes involved the upper segment of the tibia 9-11 cm in length. Before operation all the patients underwent puncture biopsy and standard chemotherapy. The lesion extent was determined by X-ray, CT and MRI, and then the size of the prosthesis was determined. The length of the prosthesis was 1-2 cm longer than that of the excised bone. After operation the patients were given neoadjuvant chemotherapy for 2 courses and they could walk with the help of a special brace 4 weeks postoperatively. Results All the 5 patients had a successful semi-joint prosthesis replacement exceptone patient who had a skin flap necrosis, and the wound healed after a flap grafting. The remaining patients had their wounds healed by first intention. The12-36 months’ follow-up revealed that all the patients had no metastasis or recurrence of the tumor and they were living and well except one patient who died oflung metastasis 8 months after operation. Conclusion The semi-joint prosthesis replacement in the limb salvage surgery for maligant tumors around the children’s knees has advantages of avoiding a damage to the normal osteoepiphysis, stabilizing the knee joint, and facilitating elongation of the limb in future. Italso creates the condition for total knee replacement in adults.