The primary objective of this study is to estimate of the mesiodistal crown diameters of the unerupted permanent successors derived from the mesiodistal crown diameters of the deciduous teeth in Korean population. The subjects were 54 indiciduals (twenty nine boys and twenty five girls) with normal occlusion aged 6 to 13 years. The mesiodistal crown diameters of the deciduous and the successional permanent teeth were measured from the longitudinal dental cast models using the shding calipers (Mitutoyo Co.) From the study, the results are as follows, 1. Sex differences of mesiodistal crown diameters were less in the deciduous teeth, but male were more than that of female in the successional permanent teeth. 2. The mesiodistal crown diameters of the deciduous central incisors, lateral incisors, canines were smaller than that of the successional permanent teeth and the deciduous 1st molars, and 2nd molars were more larger than that of the successional permanent teeth. 3. Size differences between sum of the mesiodistal crown diameters of central meisors and lateral meisors in the decidous teeth and the successional permanrnt teeth were 7.20 ± 1.79 mm in upper, 5.38 ± 1.64 mm in lower and that of canine, 1st molar and 2nd molar in the deciduous teeth and the successional permanent teeth were 0.56 ± 1.19 mm in upper, 2.22 ± 1.19 mm in lower. 4. In male, the correlation coefficients between the upper deciduous central incisor and the successional permanent tooth (r=0.57) and in female, the correlation coefficients between the upper deciduous 1st molar and the successional permanent tooth (r=0.67) appeared the highest. 5. The regression constants were determined to estimate the mesiodistal crown diameters of the unerupted successional permanent teeth.