To investigate the prevalence of obesity and distribution of body mass index (BMI) in school children of four ethnic groups in Urumqi, Xinjiang, China, Methods A total of 55 508 school children of Han, Hui, Uygur and Kazak nationalities aged 8-18 years were selected by a cluster sampling from a districts of Urumqi City for anthropometric measurement and demographic survey. Prevalence of obesity and overweight and distribution of body mass index (BMI) by gender, age, and nationality were analyzed and compared. Cutoff points of BMI for defining obesity and overweight were based on the proposal set by the Working Group on Obesity in China (WGOC) to assess age-, gender- and nationality-specific prevalence of obesity and overweight. Results Prevalence of obesity was 5.34%, 6.78%, 3.39 %, and 1.22% for boys and 2.61%, 1.83%, 1.78%, and 1.40% for girls of Han, Hui, Uygur and Kazak nationalities, respectively. Prevalence of obesity tended to decrease with age overall, whereas that of overweight increased with age in Han children. Conclusions Prevalence of obesity in school children in Urumqi varies with their nationalities and is lower than that of an average national level and a level of western countries. Obesity is more prevalent in boys than in girls of Urmuqi overall, which is just the opposite in Kazak children. Han boys and Hui girls have the highest prevalence of obesity and Kazak boys and girls have the lowest ones. Prevalence of obesity decreases with age, but that of overweight shows a different trend. To investigate the prevalence of obesity and distribution of body mass index (BMI) in school children of four ethnic groups in Urumqi, Xinjiang, China, Methods A total of 55 508 school children of Han, Hui, Uygur and Kazak nationalities aged 8-18 years were selected by a cluster sampling from a districts of Urumqi City for anthropometric measurement and demographic survey. Prevalence of obesity and overweight and distribution of body mass index (BMI) by gender, age, and nationality were analyzed and compared. Cutoff points of BMI for defining obesity and overweight were based on the proposal set by the Working Group on Obesity in China (WGOC) to assess age-, gender- and nationality-specific prevalence of obesity and overweight. Results Prevalence of obesity was 5.34%, 6.78% 3.39%, and 1.22% for boys and 2.61%, 1.83%, 1.78%, and 1.40% for girls of Han, Hui, Uygur and Kazak nationalities, respectively. Prevalence of obesity tended to decrease with age overall, but that of overweigh t increased with age in Han children. Conclusions Prevalence of obesity in school children in Urumqi varies with their nationalities and is lower than that of an average national level and a level of western countries. Obesity is more prevalent in boys than in girls of Urmuqi overall , which is just the opposite in Kazak children. Han boys and Hui girls have the highest prevalence of obesity and Kazak boys and girls have the following ones. Prevalence of obesity decreases with age, but that of over weight shows a different trend.