In the subjects of Down's syndromes from 3 to 22 years of age (48 males, 26 females), we examined the appearance rate of morphological abnormalities about the giant teeth, microdont and peg-shaped teeth. 1. The appearance rate of morphological abnomalities of the permanent teeth crown for the male was 57.8% (26/45), for the female 52.2% (12/23), very high frequency. The sexual difference was not almost acknowledged. 2. It was showed that the frequency of morphological abnormalities about the present permanent teeth for the male was 17.4% (135/775), for the female 13.6% (57/419), in upper jaw the male 19.3% (70/362), the female 17.5% (38/217), in lower jaw the male 13.7% (65/413), the female 9.4% (19/202). The sexual difference in the maxilla was hardly showed. In the appearance rate of the mandible the male was higher than the female, the rate was higher in the upper jaw than in the lower jaw through both sexes. 3. In the morphological abnormalities of the tooth crown, the giant teeth appeared only the male 0.3% (2/775), the microdont teeth the male 16.5% (128/775), the female 11.9% (50/419). Peg-shaped teeth appeared in the male 0.6% (5/775), in the female 1.7% (7/419). 4. The tooth of high frequency about morphological abnormalities was in the upper lateral incisors in both male and female subjects, the male 52.8% (28/53), the female 51.4% (19/37), after this the upper second premolar, the lower second premolar and the upper and lower first premolar.