Introduction:Regular physical activity has positive effects on physical, mental and social aspects of individual and community health. Considering the prevalence of noncommunicable diseases such as primary hypertension, osteoporosis and cardiovascular diseases for which sedentary lifestyle is a responsible factor, health policy makers have planned to encourage people to do more physical activities. Development of beneficial health habits during childhood and early adolescence are very important because the behaviors, which start at adolescence tend to continue into adulthood, too. This study aimed to determine the effect of educational intervention on physical activity-related knowledge, attitude and behavior of the first grade students in male high schools of Bavanat in 2010.Materials and Methods:In this prospective experimental study, after selecting two qualified schools from five high schools, all students of one school (two classes including 42 individuals) were randomly assigned to the experimental group and all the students of another high school (two classes including 43 individuals) were randomly assigned to the control group. The data gathering tool was a 4-section questionnaire which included demographic questions, knowledge- and attitude-related questions and the questionnaire of physical activity performance. The first three sections of the questionnaire were the researcher-designed and validated by face and content validity. Test-retest and internal consistency (Cronbach's Alpha) methods were used to determine the reliability of knowledge questionnaire and attitude questionnaire, respectively. For the measurement of physical activity behavior, a self-reporting questionnaire (valid and reliable Garcia scale) was used in this study. After conducting the pretest, the educational intervention was done for the experimental group. Post-tests were conducted immediately and 1 month after intervention. The collected data were analyzed using SPSS18 statistical software (independent t-test, paired t-test, chi-square, Mann-Whitney test and repeated measures ANOVA). The participants voluntarily and consciously participated in this study.Results:The findings indicated no significant differences between the groups in terms of mean scores of knowledge (P = 0.934), attitude (P = 0.155) and behavior (P = 0.387) before the intervention. There was a significant difference between mean scores of knowledge in the immediate follow-up (P< 0.001) and 1 month after intervention follow-up (P < 0.001), and also between mean scores of attitude immediately (P < 0.001) and 1 month after intervention (P = 0.01) follow-ups in the experimental group compared with the control one. Mean scores of physical activity 1 month after the intervention in both experimental and control groups significantly increased (P < 0.001, P = 0.01, respectively), but the mean scores of physical activity between two groups were not significantly different (P = 0.390).Discussion and Conclusion:The results indicated the effectiveness of educational intervention, consequently increased knowledge and improved attitude of students, in the experimental group compared with the control one in terms of physical activities; thus, physical activity behavior of intervention group increased. Although the mean scores of physical activity in the control group significantly increased, other studies should be done which can control and consider confounding variables.