Objective To analyze the effectiveness of free vascularized fibula grafts (FVFG) for extensive bone defects after resection of lower limb malignant bone tumors. Methods Between November 2015 and July 2018, 15 cases of lower limb malignant bone tumors were treated. There were 12 males and 3 females with an average age of 12.3 years (range, 9-21 years). There were 11 cases of osteosarcoma and 4 cases of Ewing’s sarcoma. The tumor located at middle femur in 8 cases, lower femur in 4 cases, and middle tibia in 3 cases. The disease duration was 2-6 months (mean, 3.2 months). The tumor was completely removed, and the length of the bone defect was 8-23 cm (mean, 17.7 cm). The bone defect was repaired by FVFG, and combined inactivated tumor bone was used in 8 cases of femoral bone defect. Results The average operation time was 280 minutes (range, 180-390 minutes). The average blood loss was 310 mL (range, 200-480 mL). All incisions healed by first intention. All patients were followed up 2-24 months with an average of 14.5 months. Bone healing achieved in all patients at 9-18 months (mean, 12.3 months) after operation except 1 patient which was followed up only 2 months. The fibula grafts had active metabolism and the average bone metabolism score was 184 (range, 111-257) in effected side and 193 (range, 127-259) in contralateral side. There was no difference between bilateral sides. The average Enneking score was 24.6 (range, 20-30) at last follow-up. No ankle instability or paralysis of common peroneal nerve occurred. Conclusion FVFG appeared very efficient in repair of extensive bone defect after resection of lower limb malignant bone tumor.