Abstract: Objective To study the relationship between children obesity and diet,physical activity and the rs12970134 polymorphism of melanocortin 4 receptor gene(MC4R) in Chinese children,and to provide evidence for prevention and control of obesity.Methods Totally 2 030 students aged 7 to 18 years were selected from 9 schools in Dongcheng and 5 schools in Haidian district,Beijing.Physical examination,questionnaire investigation and detection of the rs12970134 polymorphism were performed for all subjects.Results Mutation rate of the rs12970134 polymorphism was 21.1% in the subjects.No significant association was found in the mutation genotype between obese group and the control group(P=0.222,odds ratioOR=1.13,95% confidence intervalCI:0.93-1.36).The rs12970134 polymorphism and sedentary behavior had significant combined effect on obesity(OR=1.69,95% CI:1.13-3.31,P=0.015).Conclusion Sedentary behavior might modify the effects of the MC4R rs12970134 variation on obesity in Chinese children.