This paper presents a systematic analysis of the impact of socioeconomic status (SES) on overweight and obesity in China and investigates how and why the SES-obesity gradient differs with age. Using a longitudinal sample drawn from the China Health and Nutrition Survey (CHNS), I find that body mass index (BMI) is positively associated with SES during early childhood but becomes inversely related to childhood SES as children age into adulthood. Estimation results show that children from low SES families are less likely to be overweight or obese than their median and high SES peers. The results from subsamples stratified by living area reveal that the SES gaps of obesity are generally larger for urban residents than rural residents. Females are significantly less likely to be overweight than males in China. The SES during childhood has independent effects after controlling for respondents’ contemporaneous SES. The relationship between the contemporaneous SES of a male adult and his chance of being overweight or obese is significantly positive, while the contemporaneous SES of a female adult is negatively related to her chance of being overweight or obese.