The purpose of this study was to identify characteristics related to self assertiveness in teenage girls, and to identify the relationship between the self assertiveness and sexual experiences in teenage girls in Korea. The subjects for this study were 12,733 girls from an accessible population of 19,000 girls who were a multi-stage cluster sample from a population of 1,988,902 girls attending to 4,684 schools in the seven large cities and nine provinces of Korea. The response rate was 68.9%. Data were collected by mail from October 2 to October 28, 2000. A structured questionnaire of 125 items which included measurement of general characteristics, sexual experiences, and self-assertiveness was used. The sexual experiences were defined as dating, holding hands, putting arms on the shoulders, light kissing, French kissing, touching breasts, touching genitalia, and coitus. The self assertiveness measurement was developed by S. B. Chang et al.(2000) and has a Cronbach's alpha of .6031. Data was analyzed with SPSS 10.0 Program using descriptive statistics, reliability, and t-test. The results of this study are as follows; 1. The subjects were from 9th to 11th graders and 42.7% answered that they followed their partner's request. The range for the self assertiveness score was 7-21 out of a possible range of 7-21. The group of girls who were in vocational schools, lived away from family or in rural areas, attended night school, took part in drinking, smoking, and glue inhalation, who had cyber sex or phone sex or were exposed to pornography, and who had run away from home showed significantly lower self assertiveness scores than those without these characteristics (P<.05). 2. The group which had experience in dating(t=2.379, P=.017), French kissing (t=5.425, P=.000), touching breasts (t=8.637, P=.000), touching genitalia (t=6.057, P=.000), and coitus(t=6.057, P=.000) showed significantly lower self assertiveness scores than the group which had not had these sexual experiences. But there was no difference in the self assertiveness scores between the group which had experience of holding hands, light kissing, and using contraceptives compared to the group which did not. It can be concluded that the group which had delinquent behavior showed lower self assertiveness, and the lower self assertiveness led to unwanted sexual experiences. It is suggested that self assertiveness training be provided for the group with delinquent behavior as a first priority, and then analyze of the process of self assertiveness in relation to sexual experiences.