Abstract: Objective To explore the relationship of breastfeeding with childhood body mass index (BMI),overweight and obesity based on data collected by the China Family Panel Studies,and to explore whether breastfeeding can prevent childhood obesity for childhood obesity prevention.Methods Data were collected from 2 388 children aged ≥5 years.The duration of breastfeeding was first treated as a continuous variable and subsequently dichotomized into ever vs.never, ≥6 months vs.<6 months, ≥8 months vs.<8 months and ≥12 months vs.<12 months.A propensity score was calculated for various confounding factors via every dichotomization.We compared whether there is a significant difference in BMI regarding breastfeeding status.We also performed quantile regression on the upper quantiles of the BMI distribution.Results We did not find any statistically significant effect of breastfeeding on reducing body weight after adjusting for the propensity score.Instead,our result indicates that too long duration of breastfeeding can increase the risk of being overweight (odds ratio=1.42,95% confidence interval:1.07-1.89;P=0.02).Quantile regression revealed no beneficial effect of breastfeeding among children with extreme BMI.Conclusion Our research finds no significant protective effect of breastfeeding in reducing BMI or the risk of overweight and obesity among children in China.