The current study aimed at identifying the effectiveness of cognitive conflict strategy in improving academic achievement and modifying sex education misconceptions in the Science course among intermediate second-grade students in Jeddah. Based on the cognitive conflict strategy, a teacher guide was prepared for teaching Family Planning and Reproduction Chapter. In addition, an achievement test and a misconception scale of sex education are prepared. The sample consisted of 56 students equally distributed to an experimental group taught using the cognitive conflict strategy and a control one taught using the traditional method. Results revealed that there were statistically significant differences between the average scores of the experimental and the control groups in the achievement test and the misconception scale of sex education, favoring the experimental group. The study recommended holding training courses, workshops, conferences and synopses for Science teachers and supervisors on the use of cognitive conflict strategy.