Substantial evidence documents grandchild care is associated with self-reported health, life satisfaction, and depressive symptoms among middle-aged and older adults. However, little is known about the relationship between grandchild care and biological aging, especially in China, which emphasizes the unique cultural value of family. The current study sheds light on the biological consequence of grandchild care by examining the link between grandchild care and biological aging among middle-aged and older adults in China, and how gender and spousal involvement in caregiving affect this link. In a representative sample of Chinese adults aged 45-80 from the third wave of China Health and Retirement Longitudinal Study in 2015 (n = 3,384), we calculate biological age using Klemera-Doubal Method, and Ordinary Least Square models are used to examine the correlation between grandchild care and biological aging. High intensity of involvement in grandchild care is related to biological aging, and caring for grandchildren alone predicts greater biological aging. Compared with grandfathers, grandmothers lose more from grandchild care regardless of whether their husbands are involved in the care. Providing grandchild care should be a way to cope with age-related role discontinuity or loss, rather result in extra stress or burden for grandparents. Reducing the intensity of caregiving or increasing family support may attenuate the extent of biological aging.