The objectives of this study were to identify predictable factors affecting risky sexual behavior and to evaluate the effectiveness of a risky sexual behavior prevention program among boys one and two months after the program. The Information-Motivation-Behavioral skills (IMB) model program was carried out among 74 adolescent boys aged 10-13 years and 74 of their parents in Phitsanulok Province, Thailand. Repeated analysis was employed to analyze the program's effectiveness. After the program, mean scores for comfort in talking about sex, self-efficacy, and skills in condom use increased significantly (p