This study aims to determine whether assertiveness predicts restriction anxiety, separation anxiety and expectation of rejection, which are the sub-dimensions of Separation-Individuation Test of Adolescence. The study was conducted with high school students in a Central Anatolian city during the second semester of the 2018-2019 academic year. A total of 488 students, 324 girls and 164 boys, participated in the study. Regression, Pearson correlation and t test were used in the analysis of the data obtained T-test results indicated that sex significantly differentiated the scores of separation anxiety while it did not differentiate the scores of restriction anxiety and expectation of rejection. Correlation analysis showed that there was a negative relationship between assertiveness level of adolescents and level of separation-individuation (restriction anxiety, separation anxiety and expectation of rejection). Regression analysis revealed that assertiveness was the negative predictor of separation anxiety, expectation of rejection and restriction anxiety. These results indicate that, for a healthy development in adolescence, a critical period in personality development, becoming assertive and escaping passiveness can have a significant contribution to the personality development of the individual. The findings of the research were discussed and interpreted within the framework of the literature and suggestions were made to researchers and practitioners.