The purpose of this study was to clarify the relationship between the crown size and the enamel thickness of the upper primary teeth. The materials were 59 labio (bucco) lingually nondecalcified ground specimens of the upper primary 894teeth which were extracted from Japanese children. The measurements for the size of the teeth were taken from the mesiodistal diameter, the labio (bucco) lingual diameter, and the rectangle. The measurements of the enamel thickness were taken at the maximum protrusive point of the crown, and from the thicknest diameter of the enamel on the labial (buccal) surface and lingual surface.