The current study investigated the relation between spatial ability and children's math performance, as well as the potential mechanisms underlying this relation including numerical magnitude knowledge, understanding of arithmetic operations, and word-problem representation. A sample of 221 Asian sixth graders (age 11, 129 boys) completed a series of tasks measuring their spatial abilities, math performance, the potential mechanisms underlying the space-math relation, along with other potential confounding factors such as intelligence and working memory. Results indicated that spatial ability was significantly associated with math performance beyond the effects of controlled variables. Additionally, numerical magnitude knowledge, understanding of arithmetic operations, and word-problem representation were found to partially mediate the relation between spatial ability and math performance. These findings clarify the mechanisms underlying the space--math relation.