Adolescence is often regarded as a transitional stage between childhood and (emerging) adulthood. Adolescence also is viewed as a stage with heightened risks to a healthy development (Adams, Gullotta, & Markstrom-Adams, 1994; Roberts, Lewinsohn, & Seeley, 1996). Erik Erikson (1968) postulated that identity formation is the most important developmental task during adolescence. Researchers also suggested that a more confirmed sense of identity is more likely to lead to positive mental health (Erikson & Erikson, 1950; Marcia, 2002) and optimal psychological functioning (e.g., self-esteem; well-being) (Adler, Lodi-Smith, Philippe, & Houle, 2016; Lillevoll, Kroger, & Martinussen, 2013; Schwartz et al., 2011; Waterman, 1992). Besides, self-esteem is also an important developmental issue in the field of adolescence, reflected to as self-worth or self-image, is the global evaluative dimension of self (Harter, 2006). Researchers have been interested in self-esteem changes during adolescence in the long run (e.g., Harter, 2006; Kling, Hyde, Showers, & Buswell, 1999). For example, researchers generally found that self-esteem often decreases when children make the transition from elementary school to junior high school (Hawkins & Berndt, 1985; Twenge & Campbell, 2001). Robins and his colleagues also indicated that this decrease in self-esteem might take place during life transitions (e.g., from elementary school to junior high school, from junior high school to high school; from high school to college) (Robins, Trzesniewski, Tracy, Gosling, & Potter, 2002). In the past, several studies demonstrated that low self-esteem can influence adolescent mental health and psychopathology (e.g., depression, eating disorders, delinquency…) (Kuhlberg, Peña, & Zayas, 2010; Harter, 1993; Unger, Kipke, Simon, Montgomery, & Johnson, 1997; Usher, Zahn-Waxler, Finch, & Gunlicks, 2000). Numerous studies investigated adolescent self-esteem in various ways (e.g., Baldwin & Hoffman, 2002; Prinstein & Dodge, 2010; Roustit, Campoy, Chaix, & Chauvin, 2010), but the role of self-identity in self-esteem changes during adolescence has relatively received little attention. Based on Erikson’s belief that a more confirmed sense of identity is more likely to lead to positive psychological adjustment (Erikson & Erikson, 1950) and help individuals ensure the direction, purpose, and meaning of life, this study proposed that all aspects of identity firmness are key predictors to self-esteem at all substages of adolescence. In addition, as different stages highlight different needs for the various aspects of identity, this study hypothesized that, in early and middle adolescence, social and image identity importance would significantly predict self-esteem; while in late adolescence, only personal identity importance would be significantly related to self-esteem. To examine the above hypotheses, four stepwise regression analyses of four serial studies on self-esteem using the aspects of identity importance and firmness as predictors were conducted in three different age levels of adolescents. Study 1 (N = 1,285 participants from junior high, high school, college students) generally supported the hypothesis that identity firmness could predict adolescent self-esteem. That is, the results of Study 1 showed that there were different patterns on identity importance in predicting self-esteem among three adolescent stages, but same patterns on identity firmness (i.e., personal identity firmness, social identity firmness) in predicting self-esteem among three adolescent stages. Study 2 (N = 203 participants from three different colleges) successfully replicated the findings of Study 1. In Study 3 (N = 185 undergraduate students) and Study 4 (N = 146 undergraduate students), the measurement of identity importance and identity firmness was revised. Again, the findings were consistent with those of the previous two studies. In sum, this study suggested identity importance and identity firmness are crucial factors contributing to adolescent self-esteem.