Childhood obesity in China has been increasingly cited as a major public health issue in recent decades. The effect of grandparents on grandchildren's weight outcome is under-analyzed. Using data from the China Health and Nutrition Survey, we examine the effect of grandparents' coresidence on childhood weight outcome with a sample of 2-13-year-old children in China. We use instrumental variables to address the potential endogeneity of grandparents' coresidence. We show that the effect of grandparents' coresidence on childhood weight outcome is significantly positive. Grandparents' coresidence affects a grandchild's weight outcome through changes in dietary patterns and physical activity. The effects on dietary patterns exist in urban areas and significantly lower in rural areas. Grandparents' coresidence decreases physical activity more in rural areas than in urban areas. Furthermore, the effects of coresidence on protein intake and physical activities of children above 6 are significantly higher for males than females. A robustness check, including an ordered logit model with a body mass index category and estimation with additional data, validates our findings.