PURPOSE: The purpose of this study was to investigate the relationship between high school students' self-differentiation, family function and their level of stress. METHOD: A questionnaire which consisted of questions on general characteristics of the high school students, and 36 questions on self-differentiation, 17 questions on family function, and 37 questions on level of stress was used to collect the data. Participants were 201 second grade high school students from Bucheon City. Descriptive statistics, T-test, ANOVA, correlation and multiple regression were used with SPSS 10.0 to analyze the data. RESULTS: The mean scores for self-differentiation, family function, and levels of stress were 3.27, 3.39, and 2.61 respectively. The relationship between self-differentiation and level of stress revealed a significant negative correlation. The relationship between self-differentiation and family function showed a significant positive correlation. The relationship between family function and stress level showed a significant negative correlation. CONCLUSION: The results of the study show that variation in level of stress was related to family regression, recognition/emotional function, family projection, role recognition and emotional support and emotional cutoff which together explained 40.9% of the variance in level of stress.