Introduction: Nowadays, obesity among children is rapidly increasing as a public health issue. Early detection of overweight and obesity in children during their school years is an important prevention strategy. The purpose of this study is to determine the prevalence of obesity and its relationship with anthropometric measures and lifestyle factors. Materials and Methods: This cross sectional study was conducted between October 2010 and June 2011. Samples of students were selected from eight elementary schools in Shahryar city using multi-stage cluster sampling. A multi-section questionnaire, which consisted of three sections, including data on demographics and anthropometrics, physical activity and the 24 recall was used. Results: Totally, 325 students aged 6-12 years (49.2% boys and 50.8% girls) took part in the study. Mean age of participants was 8.53 ±SD = 1.61. Results revealed that the mean daily energy intake in obese children, compared to healthy weight children, was 2381/74±308.95 versus 1564.52±160.567 (P<0.001). Multiple logistic regression showed that the daily energy intake, duration of night sleep, meal during passive physical activity and father body mass index were significantly related to overweight & obesity (P<0.001). However there were no significant relationships in terms of age, sex, leisure time activities and birth order in this age group of participants. Conclusion: There are various risk factors associated with obesity and overweight in children, based on which health education programs should be designed and implemented to help control obesity and overweight in this age group.