1. This study of breast development of Korean girls from psychological point of view was made as a part of overall studies of developmental pattern of Korean boys and girls. 2. The subject girls are from attached high-school, Seoul National University. numbered 507. and ages ranged from 10-5 to 19-7. A doctress interviewd each student and checked the developmental stage of breasts according to Stratzs classification. 3. The stage of budding breast begins about from 10-5. and maturedbrast begins around 16-3 by M or Mdn values. Almpst all of the girls reach the stage of ma tured breast about 18years old. As sampis is a little biased. overgeneralzation is warned. 4. There are great individual differences at the rate of breast development. so we can conclude that sexual development of Korean female has wide individual differences. For example. eraly maturation brgins at 13-9. later maturation begins at 19-7. thus 5-10 differences in maturing ages. 5. In comparing with the American and Japanese girls. Korean girls teach the maturity a little earlier than Japanesegirls, but two pro three years late than American girls. we can assume that there is some anthropological and cultural differences in the differential rate of breast developments.