Os subfibulare is a accessory bone arround the lateral maleolus, but it is different from many other accessory bones in foot and ankle in its development related to trauma and frequent symptoms of ankle instability. Accessory bone is a developmental anomaly which fails to unite to main secondary ossification center and usually asymptomatic. But os subfibulare is developed by recurrent ankle sprains in younger age or non-union of avulsion fracture of anterior talofibular ligament and causes instability of ankle. We experienced two cases of symptomatic os subfibulare in adolecents and report them with review of literatures.