To investigate the impact of grandparent's co-residence and dietary knowledge on the overweight and obesity risk among school-age children in China.Data of 790 children aged 7-15 years were retrieved from the China Health and Nutrition Survey (CHNS). Co-residential status was evaluated using household and individual ID. Dietary knowledge was assessed using 17 dietary statements, which was scored by participants. Three principal factors were extracted by factor analysis. Logistic regression analysis was applied to examine the influence of grandparent's co-residence and dietary knowledge on the weight status of the child.The overweight and obesity incidence of the children increased significantly when they co-reside with two grandparents (OR=1.447, 95CI: 1.001-2.090, p=0.049). Mother's dietary knowledge regarding to protein-related food choices (PT) scored higher was associated with the increased risk (OR=1.270, 95CI: 1.059-1.523, p=0.010). For grandmother's dietary knowledge, the higher score in balanced food consumption (BF) was associated with the decreased overweight and obesity risk (OR=0.456, 95CI: 0.294-0.706, p<0.001). Grandfather's dietary knowledge in BF showed the similar result (OR=0.454, 95CI: 0.271-0.759, p=0.003).Living with two grandparents is associated with higher overweight and obesity risk, but grandparents with an adequate knowledge in the balanced food consumption may decrease the risk.© 2022 Walter de Gruyter GmbH, Berlin/Boston.