Purpose: The purpose of this study was to investigate prevalence and factors influencing constipation in school age children in order to help prevent and manage this problem. Methods: The participants were 618 elementary school students (4th, 5th, and 6th grade) selected from five elementary schools. Data were gathered using questionnaires and analyzed using $x^2$ test, independent t-test and binary logistic regression with the PASW 18.0 program. Results: The prevalence of constipation was 15.2% for these school age children. Factors influencing constipation in this population were high stress (OR: 5.77), daily intake of meat (OR: 5.42), more than 3 hours using the internet (OR: 4.50), more than 3 hours of private academy classes (OR: 2.83), obesity (OR: 2.34), and negative perceptions of school toilet environment (OR: 2.26). Conclusion: Psychological factors and physical activities have an important effect on constipation in school age children. Thus, there is a need to educate both children and their parents on prevention of constipation by decreasing stress, decreasing meat intake, increasing active movement and increasing positive perception of school toilet environment.