It is very important for elementary school students in sixth grades who are reaching the age of puberty to acquire right knowledge and desirable attitude toward sex so that can overcome psychological instability caused by physical growth, be responsible for their behaviors and lead happy lives. Therefore, the purpose of this study was to find out the effects of sex educatin in the population of primary school students. The subjects of this study were 767 six-grades in one private and two public elementary school in Seoul. The research tool was a questionnairebased on a literature review. The sex education given to the subjects included 50 minutes lecture. The research methodology included data collection done before and the sex education was given to the subjects, to assess their knowledge of, and attitude toward sex, one week later after the sex education was given to assess change in knowledge of, and attitude to, sex. The data was analyzed through pc-SAS program. real numbers, percentage, 1-test, and ANOVA were utilized. The results of this study are as follows : 1. After sex education, the score of sixth grade elementary students’ sex knowledge was higher than before(t=11.92, p=0.0001) 2. After sex education, the score of sixth grade elementary students’ sex attitude was higher than before (t=2.08, p=0.0373). From the above findings, it can be said that sex education given to the children reaching the age of puberty significantly influences their knowledge of, and attitude toward. Therefore, for children to have the sound knowledge and positive attitude of sec, it is suggested that sex education should be included in their curriculum in order that systematic sex education be practiced.