["Child &Family Social Work, EarlyView. ", "\nABSTRACT\nParent–adult children's conflict is a big challenge faced by grandparents during grandparenting, but its impacts on the mental health of the grandparents remain unclear. This study aimed to explore the roles of parent–adult children's conflict on the associations between caregiving burden, social capital and uncertainty stress among older adults involved in grandparenting. We conducted a cross‐sectional survey on 975 older adults engaged in grandparenting in Hangzhou, China. The findings indicated that 24.8% (n = 242) of the grandparents reported high uncertainty stress, and 6.4% (n = 63) reported general or profound parent–adult children conflict. Parent–adult children's conflict was significantly correlated with caregiving burden (r = 0.13, p < 0.05) and social capital (social participation: r = −0.17, p < 0.05; social network: r = −0.12, p < 0.05), strong and independent predictors of uncertainty stress in grandparents (p < 0.05). Interactive effects analysis indicated that buffering effects of social capital on uncertainty stress may vary with the degrees of caregiving burden and parent–adult children's conflict. Overall, the buffering effects of social capital on uncertainty stress were likely to be more pronounced when parent–adult children's conflict was general, and the caregiving burden was high. Our findings may assist in comprehensively understanding the social determinants of the mental health of grandparents involved in grandparenting.\n"]