This study was undertaken to obtain the data for age determination following the calcification degree of the third molar in the point of forensic odontology. The orthopantomograms of 1399 male and 2517 female ranging from 7 to 22 years of age were studied. The developmental state of the third molar was divided into 12 stages following the calcification degree and the mean ages of stage was obtained and statistically analyzed. The results were as follows : 1. There was remarkable correlation between the development of the third molar and age. 2. There was no significant developmental difference between the right and left third molar. 3. The development of the third molar presents difference between the male and female in the crown formation stages, but it was retarded in female than male in the root formation stages. 4. At the age under 20 years, we can determine the age within the range of * 2 years by means of orthopantomogram.