The present study aims at revealing the effects of the unique interrelationship of identical twins on their personality formation. Questionnaires, interviews and diary survey were administered to 100 pairs of junnior and senior high school twins; and questionnaires to their mothers. They were mainly intended to investigate the attitudes and feelings held with respect to each other. In order to compare, one of the questionnaires was also given to non-twin pupilps of the same schools, who were about 400 in number. The results were as follows: (1) The awareness of partnership between twins was compared to that of siblings and close friends. As for females, relations of twins were tighter than those of siblings and friends. For males, the total score was the same, but partnership of twins was characterized by emotionality and indifferetiatedness, while, friendship was characterized by co-participation and task-orientatedness. This tendency of indifferentiatedness seems to be confined to identical twins. (2) The relationship of female twins were much closer than those of male twins, but no marked difference was found between grades. (3) Most paires of twins held positive attitudes toward their present status (twinship). (4) Attitudes of mothers were found to be correlated with interrelations of adolescent twins. In rearing practices when parents tried to deal with twins as similarly as possible, the relationshilp was tighter than when they followed, the principle of independent treatment.