Background: Childhood obesity considers a global health problem; 100 million health initiative was launched for early detection of non-communicable diseases such as obesity. Aim: This study aimed to evaluate the effect of evidence based obesity prevention program among primary school students according to 100 million health initiative. Research design: A quasi experimental design was carried in this study. Setting: This study was conducted at seven Governmental Primary Schools in Benha City. Schools selected randomly one from each sector from total 108 schools that distributed into 7 sectors. Sample: Systematic random sample was used to select 180 primary schools students.  Tools of data collection: Three tools were used. I: A structured interview questionnaire which included demographic characteristics of the primary school students, family and medical history, knowledge regarding obesity and 100 million health initiative. II: Practices of primary school students regarding prevention of obesity. III: Attitude scale regarding prevention of obesity.  Results: 10.0% of primary school students had good knowledge regarding obesity and 100 million health initiative pre-implementation of program which improved to 75.0% post- implementation of program, 9.4% of them had satisfactory practices regarding prevention of obesity pre-implementation of program which improved to 72.8% post-implementation of program and 27.8% of them had positive attitude regarding prevention of obesity pre-implementation of program which improved to 82.2% post-implementation of program. Conclusion: Evidence based obesity prevention program had a significant positive effect in improving knowledge, practices and changing attitude of primary school students. Recommendations: Continuing evidence based obesity prevention program for primary school students to increase their knowledge and practices to prevent obesity.