This study aims to find the personality development of the main character in the Wonder novel, August Pullman. The objective of this study is to show how August’s interpersonal relationship depicted in the novel and influenced his personality development. The theory of Interpersonal Relationship from Harry S. Sullivan is used to analyze how August’s social interaction with others affect his personality development. This study found that someone’s personality development can be seen through the relationship between individual and others. In addition, to find out the factors of personality development, this study uses Elizabeth B. Hurlock’s theory of Personality Development. Some factors which are responsible for someone’s change are changes in environment, changes in social pressure and changes in self-concept. Based on the analysis, August’s personality in Wonder novel is described as an unconfident, weak, and has limit social interaction developed into confident, brave, and independent kid