The purpose of this study was to test Bowen`s family theory. Adolescents are in a vulnerable stage of mental health and have many mental health problems and psychosis. In Bowen`s theory differentiation of self is related to mental health suatus. So this study was designed to identify the relation between differentiation of self and mental health in adoleescents. Therefore the point of this study was to contribute to the theoretical understanding of the relationship between these two concepts and eventually to the enhancement of theoretically directed nursing care. The subjects for this study were 299 high school students attending a high school in Seoul : the number of first year, second year and third year students included was 101, 100 and 98 students respectively. The data were collected during the period form from March 31, 1992 to April 4, 1992. For measuring the differentiation of self as described by Bowen a questionnaire by Je (1989) was used. A simplified Mental Diagnostic Test adapted for Koreans from Derogatis, et al(1976) by Lee(1986) was used to measure the level of mental health. The data were analyzed with the SPSS Program using descriptive statistics, Pearson Correlation Cofficients, ANOVA and Stepwise Multiple Regression. The results of this study can be summarized as follows : 1. The mean score for the differentiation of self scale was 101.88. 2. The mean score for the level of mental health was 99.06, which was higher than the results from other age-groups for which this tool has been used. 3. Hypothesis 1) "the higher the level of cognitive function vs emotional function, the higher the level of mental health" was supported(r=.53 p＜.001). Hypothesis 2) "the higher the level of integration of self, the higher the level of mental health" was not supported (r=.08 P＞.05). Hypothesis 3) "the lower the level of family projection process, the higher the level of mental health" was supported(r=-.30 p＜.001). Hypothesis 4) "the lower the level of emotional cutoff, the higher the level of mental health" was supported(r=-.36 p＜.001). Hypothesis 5) "the lower family regression, the higher the level of mental health" was supported(r=-.41 p＜.001). 4. By using Stepwise Multiple Regression Analysis, the main influencing factors for the five different categories of differentiation of self were identified, as follows : 1) For the first subcale of differentiation of self the "Cognitive function vs Emotional function" ; three main variables : hostility, depression, and paranoid ideation explained 41.5%( p＜.001) of the varance. 2) For the second subcale "Integration of self" ; two main variables; depression and hostility explained 9%(p<0.001) of the variance. 3) For the third subcale "Family projection process" ; three main variables hostility, paranoid ideation, and psychosis explained 15%(p p＜.001) of the variance. 4) For the fourth subcale "Emotional cutoff"; four main variables hostility, depression, obscession, and paranoid ideation explained 21%( p＜.001) of the variance. 5) For the fifth subcale "Family regression ; two main variables hostility and paranoid ideation explained 18%( p＜.001) of the variance. 5. 1) The results from the test for the differentiation of self in the area of demographic variables showed that there were significant differences related to sex(F=6.88 p＜.001), school year(F=6.08 p＜.001). and educational level of parents(F=8.87, F=5.01 p＜.001). 2) The results from the tests for the level of mental health in the area of demographic variables showed that there was significant difference related to school year(F=6.70 p＜.001). 6. The relationship of differentiation of self to the nine different categories of mental health status were as follows : The higher the degree of differentiation of self was, the higher the results of the nine different categories of mental health status were (p＜.001). 7. By using Stepwise Multiple Regression Analysis ; the main infuencing factor