The present study was designed to investigate developmental changes of self-regulation in preschool children. Seventy-five nursery school children of 4, 5, 6-year-old (41 boys and 34 girls) were required to answer a set of questions as to how they would feel and behave when they were provoked by their peers. The variables manipulated were the subjects' familiarity with their peers and uncomfortability of the conflicting situations. The results indicated that the quality of self-assertive strategy changes with age from egoistic responses to social responses. It was also found that the development of the self-assertive strategy chosen varied according to the subjects' famiriarity with their peers and the uncomfortability of the conflicting situations. Implications of these findings were discussed in terms of the development of social cognition.