Purpose: The purpose of this study was to identify the effect of sexual education on sex knowledge and attitude in elementary school students. Method: Questionnaire data were collected from 4 elementary schools located in Y-gu, Seoul. Final participants included 512 older school-age children. Results: There were significant differences in the degree of sex knowledge (t=-28.35, p<.001) and attitude (t=-18.44, p<.001) between the pretest and the post-test. The degree of sex knowledge changed significantly according to grade, sex, and experience with sex-related questions, and the degree of sex attitude changed significantly according to grade and sex. There was a significant correlation in the degree of change between sex knowledge and attitude (r=.28, p<.001). Conclusion: In this study, sex education for elementary school students effectively changed sex knowledge and attitude. Sex education is recommended elementary school students as well as adolescents in order to encourage responsibility in desirable sex behavior.