The current study was the first to investigate preschoolers’ language use while they were required to engage in social cognitive processing. By adapting Sodian et al.’s (1991) social cognitive task, the current study showed a changing pattern of relations between task performance and private speech production in 3- to 5-year-old Cantonese-speaking Chinese children. The findings suggested that language may not only passively provide children with information about the psychological world but also, in the form of semiotic mediation, actively be involved in and regulate social cognitive processing. Limitations and implications of the findings are discussed below.