Growth of the mandible of the Japanese children was investigated through amount of annual change of the mandibular length which was defined by J. E. Harris on lateral cephalometric roentgenograms. Obtaining peculiarity of growth of the mandible in Japanese, a comparative study was done on growth of the mandible between Japanese and American. The materials used in the present study were 523 sheets of cephalometric roentgenograms of children which covered a range of ages from 6 to 12 years. The mandibular length of the Japanese male sample, at each age, surpassed those of the Japanese female sample. The total growth increment from the ages of 6 to 12 years was 18.1 mm. in the Japanese male sample and 19.1 mm. in the female sample. The pronounced growth deceleration which was observed in the American sample as a preadolescent growth deceleration was irrecognizable in the Japanese sample. The similarity of the periodicity of the mandibular pattern which was elucidated between the male and female samples in the American, reported by J. E. Harris, was not observed in the Japanese. The present study should be expanded to a higher range of ages, in order to have more available informations for mandibular growth prediction.