This study is to use the longitudinal approach to investigate the development of the self concept of children and adolescents. The subjects including 648 youngsters (309 males, 339 females), were from two elementary schools and two junior high schools in Taipei area. The concept scale, followed the Tennessee self Concept Scale, was administered to the subjects three times in regular class settings. The main findings of the studying were as follows: 1. The self concept characteristics of Chinese Youngsters were sound and positive. 2. The self concept characteristics, except identity, of elementary school boys and girls did not chang with age. 3. Some self concept characteristics, such as psychological self and behavior, of junior high school students descended with age. 4. Sex differences of changes of self concept characteristics were found.