The potential influences on children's sharing decision caused by birth order and sibling age gap were examined. A third-party resource allocation task was adopted to examine five- to six-year-old children's sharing decision when they expected a protagonist to allocate resources between two recipients with different social relationships. Children with sibling age gap less than three years tended to allocate resources with a closer recipient more often than those with sibling age gap more than three years. When siblings were more than three years apart in age, children allocated resources between a sibling and a friend, first-born children were more likely to choose a sibling than second-born children. However, when siblings' age gap was within three years, second-born children, in turn, were more willing to share with a sibling than first-born children. These results were attributed to the sibling relationships and the development level of the Theory of Mind.