"Assertion" was a positive, situational behavioral trait, and related to mental health and adjustment. "Social interest" was considered to be a key factor to adjustment by Alfred Adler. To find what relationship between assertion and social interest was one of purposes of this study. What difference between male and female on assertion was also examined in this study. And finally, this study wanted to investigate the differential reactions to males´ versus females´ assertion and nonassertion. In this study, 663 subjects were sampled from the students of junior high schools. The dependent variables were the scores on assertion (included 4 subscores-i. e. interpersonal conflict resolution/negative expression, positive expression, refusal and request, and a total scale score), social interest and interpersonal attraction (included 3 subscores-i. e. likeability/consideration, competence, honesty/fair). And the independent variablse were the sex (subject´s and model´s), levels of social interest (high and low), types of situation (refusal and commendatory), model´s behavior (assertion and nonassertion). The data were analyzed by one way, two way and four way ANOVA Pearson-product Correlation, Chi-square test and Disccriminant analysis The results indicated that there was a difference between male and female on different situational assertions. There was an action between subject´s sex and levels of social interest on the score of interpersonal confict resolution/negative expression. Assertion was significantly correlated with social interest negatively. With the scores of situational assertions might be used to discriminate the levels of social interest availably, And the most important findings on differential reactions to males´ versus females´ assertion and nonassertion were quite similar to those of St. Lawrence´s et al.(1985)