For young growing children before the end of skeletal maturity, the growth activity of the grafted bone after hemimandibulectomy is not well-known. After an adolescence, such a patient may have facial deformity because the anterior growth point of the mandible is in the condylar neck. A 13-year-old boy was performed hemimandibulectomy with immediate mandibular reconstruction by fibula free flap (FFF) because of a huge ameloblastic fibroma. The authors evaluated the length of FFF on the images of computed tomography (CT) at 5 and 60 months after the operation and compared them by calculating growth rates. Five years after surgery, his facial appearance was symmetry and mandibular function was satisfaction. Although the mandibular bone in the contralateral side grew during 5-year follow-up, the vascularized FFF grafted in the child patient did not significantly grow. Moreover, spontaneous regeneration (SR) and the gradual osteosclerosis were confirmed on the left distal edge of the FFF on the CT imaging. The arrival of SR at the left distal edge of the FFF was considered a part of the reason to compensate the unchanging growth rate of the grafted FFF and contribute for the postoperative good functional and esthetic results.