Purpose: This study was done to develop a children’s sex education program for the parents of lower elementary grade students and to evaluateits effects on sexual knowledge, gender role attitude, parent efficacy for child’s sex education, and marital consistency. Methods: A quasi-experimentalwith a non-equivalent control group pretest-posttest design was used. The participants were 29 couples (58 parents, experimentalgroup=28, control group=30) from G city. The 5-week (5-session) program was developed based on ‘A theory of protection: parents as sex educators’and used the case-based small group learning method. Data were collected during July and August 2015. The characteristics of the programdeveloped in the present study were a theoretical-based, client-centered, multi-method. Results: After the intervention, the experimentalgroup showed a significant improvement in sexual knowledge, gender role attitudes, parent efficacy for child’s sex education, and marital consistency,compared to the control group. The effect sizes of the program were .64 (knowledge), .65 (gender role attitudes), and .68 (parent efficacy). Conclusion: The results of this study provided implications for the parents as effective sex educator and the role expansion of schoolhealth nurses.