Background: Overweight and obesity during childhood and adolescence are increasing as public health problem. Objective: This study aimed at examining the effect of the education program on children and adolescents’ knowledge, attitudes and behavior in relation to healthy lifestyle. Methods: This study was performed as a quasi-experimental national survey as part of the IRAN-Ending Childhood Obesity (IRAN-ECHO) program. Participants were selected from six cities of Iran. The sample size was calculated as 1264 for each city. Nutrition-related knowledge, attitudes and behavior were measured at pre- and post-intervention surveys. Results: The prevalence of excess weight (overweight/obesity) was 7.6%. The frequency of desirable knowledge after intervention (32.5%) is significantly higher than before intervention (24.8%) (p-value=0.02). The mean score of attitude about obesity complications was significantly increased from 73.09 to 74.78 after intervention (p-value < 0.05). There was significant increase in mean score of practice in relation to low consumption of unhealthy snacks after intervention compared to the baseline (difference = 1.63, p-value=0.03). The mean score of mild physical activity increased from 50.67 to 65 after intervention (p-value< 0.001). There was no significant difference between the desirable total attitude and total practice before and after the intervention (p-value > 0.05). Conclusion: Educational intervention based on WHO-ECHO recommendations can be useful for improvement the knowledge related to healthy lifestyle in children and adolescents; over time it might lead to change in attitude and in turn in improving their practice.