Objectives. The study aimed to assess the prevalence of obesity and the related behavioral risk factors among children in Beijing. Methods. Data were collected as part of a 2004 comprehensive study on obesity and metabolic syndrome among 21 198 children, aged 2–18 years old in Beijing. Four local and international references for the classification of childhood overweight and obesity were compared. Results. The combined prevalence of obesity and overweight among children, aged 2–18 years old, was 18.7%, 21.4%, and 20.1% based on the International Obesity Task Force (IOTF) reference, new World Health Organization (WHO) growth standard, and the US 2000 Center for Disease Control and Prevention Growth Charts, respectively; and it was 21.7%, according to the local Chinese body mass index (BMI) reference for school-age children (aged 7–18 years). Physical inactivity, less sleep duration, higher consumption of alcohol, snack food and Western fast food and more screen time were associated with overweight and obesity (p <0.05). Conclusions. Over 20% of children and adolescents in Beijing are overweight or obese. Effort should be made to combat the growing obesity epidemic.