Objective Previous studies in China showed large sex differences in childhood overweight and obesity (ow/ob) rates. However, limited research has examined the cause of these sex differences. This study aimed to examine individual and parental/familial factors associated with sex differences in childhood ow/ob rates in China. Design Variables associated with child weight status, beliefs and behaviors, and obesity-related parenting practices were selected to examine their sex differences and association with a sex difference in child ow/ob outcomes using logistic regression analysis. Setting Cross-sectional data analysis using the 2011 China Health and Nutrition Survey. Subjects Children aged 6–17 years (n=1,544) and their parents. Results Overall child ow/ob prevalence was 16.8%. Adolescent boys (AB; 12–17 years) were about twice as likely to be ow/ob as adolescent girls (AG; 15.5 vs. 8.4%, p<0.05). AB more likely had energy intake exceeding recommendations, self-perceived underweight, underestimated their body weight, and were satisfied with their physical activity level than AG. AG more likely practiced weight loss management through diet and self-perceived overweight than AB. Mothers more likely identified AG’s weight accurately but underestimated AB’s weight. Stronger associations with risk of childhood ow/ob were found in boys than girls in dieting to lose weight (OR= 6.7 in boys vs. 2.6 in girls) and combined maternal and child perception of the child’s overweight (OR= 35.4 in boys vs. 14.2 in girls). Conclusions Large sex differences in childhood obesity may be related to the sex disparities in weight-related beliefs and behaviors among children and their parents in China.