Background: In recent decades, the prevalence of overweight and obesity in adolescents have increased in Iran and other countries. This study aimed to assess the prevalence of obesity and its associated factors among high school students of Karaj City, Iran. Methods: A total of 700 students in different age groups were selected by random sampling method. The study samples were divided into 2 groups of case and control and their characteristics were compared. The data were collected by nutritional frequencies and remembrance of 24 hours of nutrition questionnaires. Finally, the obtained information (BMI and its percentile) was statistically analyzed with respect to samples' nutritional pattern and physical activities. Results: The prevalence of obesity and overweight in teenagers were 13.5% and 17.4%, respectively. The obesity rate in boys was higher than that in girls. The amount of daily fat, carbohydrate, protein, and calorie intake (2700 kcal) was higher in obese students compared to normal ones. Significant differences were observed between two groups in terms of fat, protein, carbohydrate intake, and calorie consumption as well as physical activity. Conclusion: The obesity prevalence in Karaj students is high and the intake of carbohydrate, fat, and calorie should be adjusted with regard to different age groups. The relevant education should be started in school and families, by considering the importance of diet reform in lowering the rate of obesity and overweight.