Background: Sexual harassment in schools is unwanted and unwelcome behavior of a sexual nature that interferes with the health and wellbeing of students, also it makes the educational environment hostile. Aim of study: the study aimed to evaluate the effect of health instructional guidelines for prevention of sexual harassment on perception of preparatory school male students at Tanta city. Subjects and method: Study design: Quasi-experimental design was used. Study setting: The study was conducted at two governmental preparatory schools. Study subjects: A convenience sample of 200 students attending the previous setting from four classes was included in the study. Tools: A structured questionnaire schedule was used to carry out this study which contained the following parts; part 1: Socio-demographic characteristics of the male students and their families, part 2: Assessment of sexual harassment experiences of male students by using Sexual Experience Questionnaire, part 3: Assessment of male students' knowledge about sexual harassment and its prevention, part 4: Assessment of used protective measures of male students toward sexual harassment, and part 5: Assessment of perception of male students toward sexual harassment. Results: The percentage of students who gained good level of total knowledge score improved from 11% to 81% pre and immediately after implementation of health instructional guidelines respectively. About 75% of them had a negative perception of sexual harassment pre health instructional guidelines and this decreased to only 17% immediately after the implementation of health instructional guidelines. Conclusion: The health instructional guidelines were effective and improved the level of perception, knowledge of the studied male students about sexual harassment were improved after implementation of health instructional guidelines. Recommendations : continuous implementation of health instruction guidelines for students and their families about sexual harassment and their role in its prevention and providing sufficient training for preparatory school male students regarding prevention of sexual harassment to enhance their perception.