The dental neonatal line of the sika deer (Cervus nippon) was identified experimentally using chronological labeling methods. In the enamel, prominent dark lines were observed under transmitted light, and the number of increments between the dark line and labeling line was almost consistent with the day-age at the time of labeling injection. Therefore, we identified the dark line as the enamel neonatal line. In the dentin, the bright line was observed under polarized light. Since the bright line corresponded to the enamel neonatal line, we recognized the bright line as the dentin neonatal line. Neonatal lines intersected with the enamel-dentin junction at approximately one-third cervical in the first molar. Using these features, it would make possible to distinguish the neonatal line in wild sika deer.