This study investigated the motivational and social-cognitive foundations (i.e., inequality aversion, in-group bias, and theory of mind) that underlie the development of sharing behavior among 3- to 9-year-old Chinese children (N = 122). Each child played two mini-dictator games against an in-group member (friend) and an out-group member (stranger) to divide four stickers. Results indicated that there was a small to moderate age-related increase in children's egalitarian sharing with strangers, whereas the age effect was moderate to large in interactions with friends. Moreover, 3- to 4-year-olds did not treat strangers and friends differently, but 5- to 6-year-old and older children showed strong in-group favoritism. Finally, theory of mind was an essential prerequisite for children's sharing behavior toward strangers, but not a unique predictor of their sharing with friends.