Free vascularized fibula is often used in orthopaedic reconstructive surgery because the fibula is a straight cortical bone, long enough, and has a long vascular pedicle. But morbidity is occurred at the donor site which may cause problems at the ankle in children. We reviewed 10 causes who had free vascularized fibula transfer at the Department of Orthopaedic Surgery, Yonsei University College of Medicine from January 1984 to June 1989. This study is attempted to evaluat the effects of fibular defect on the ankle in children and the results of treatment. Free vascularized fibula transfer was done in 5 cases due to ossifying fibroma, 3 cases due to nonunion of fracture and 2 cases due to congenital pseudoarthrosis. The average valgus deformity was 3 degrees in 2 cases in which distal tibiofibular fixation was done with one screw and 6 degrees in 8 cases in in which distal tibiofibular fixation was not done. Three cases had severe valgus deformity and secondary operation was done. The attended type of operation was distal tibiofibular fusion in 3 cases. The valgus deformity was changed after distal tibiofibular fusion from 4 degrees to 3 degrees in 3 cases. The size of fibular defect did not effect on the degree of valgus deformity of the ankle. In conclusion, fibular defect can cause valgus deformity of the ankle in children and early distal tibiofibular fusion is recommended to prevent valgus deformity of the ankle in growing child who as defect on fibular.