Supernumerary tooth is one of the abnormalities in tooth number. Supernumerary tooth result from excessive proliferation of dental lamina and incidence reports identify a range of $0.3sim0.8%$ in primary dentition, $1.0sim3.5%$ in permanent dentition with males being affected twice as frequently as females, maxilla nine times as frequently as mandible The incidence is more in anterior tooth region than in posterior region. The most common supernumerary tooth is the mesiodens, which located between maxillary central incisors. The occurrence is very rare in the incisor region of mandible and the reports on incidence is 2%. In this case, there were two supernumerary teeth in the mandibular region and we could acquire normal alignment of mandibular incisors by extraction and orthodontic treatment.