In order to live in a country, at the very least, one has to manage to put oneself in a satisfactory situation, both physically and psychologically. To know one's self is a characteristic of human beings. The theme discussed in this paper is self-assertion of young Japanese children about six to nine years old, temporarily staying with their family in the United States and studying in two schools, American and Japanese. Self-assertion research is one of the most difficult, significant, and practical issues living not only in Japan but also in America. Specifically, self-assertion expressed in behavior is observed in terms of cultural values. The causes of different self-assertion in two countries will also be studied. The author of this paper emphasizes the importance of investigating self-assertion not on1y from an outside point of view but also from the inside.