To know the correct gestational age of a neonate has an important clinical significane, because the mortalityrate, some congenital anomaly and morbidity of a neonate have a joint correlation with it's gestational age. Inorder to have a objective and correct gestational age, we have studied tooth mineralization of 46 Korean neonatesof known gestational age. All the 1st. deciduous molar appeared radiographically on 34 weeks of gestation orafter. All the 2nd. deciduous molar appeared radiographically on 36 weeks of gestation or after.