Purpose: This study was conducted to investigate the relationship of knowledge, attitudes and needs of sex education in high school students. Methods: There were 258 participants who were high school students in S high school in G city. Data were collected from October 1 to October 31, 2011, and analyzed using SPSS/WIN 19.0. Results: The mean score for sexual knowledge was 69.78, sexual attitudes was 63.66, and needs of sex education was 75.37. Sexual knowledge had positive correlation with needs of sex education (r=.17, p=.007). Conclusion: The findings of this study indicated a need to develop programs for high school students to increase appropriate sexual knowledge, and to encourage appropriate sexual attitudes, and to reflect needs of sex education.