Objective: To study the relationship between caries activity and the status of dental caries in preschool children using a caries activity test (CAT). Materials and Methods: The status of primary tooth caries in 229 children ages 3 to 5 was examined. Caries incidence, DFT, and caries severity index (CSI) were calculated. Based on the quantity of Streptococcus mutans in the oral cavity detected with Dentocult SM, a caries activity test, four grades of caries activity were classifed: Grade 0 (< 10I (10 5 - 10 6 CFU/mL), and Grade III (?10 6 CFU/mL). Results: The results showed that 79.48% children had Grade I or higher caries severity. Significant differences of caries activity were found among different grades, which were highly corerlated with caries incidence, DFT, and CSI (r = 0.22216, 0.31212, and 0.32276, respectively). Conclusions: As a CAT, Dentocult SM is a reliable method for measuring the status of dental caries in preschool children. It is also a valuable tool in the prevention and treatment of dental caries.