Objective: To evaluate the genotypic diversity and some virulence traits of Streptococcus sobrinus (S. sobrinus) isolated from caries-free children and children suffering severe early childhood caries (SECC). Methods: S. sobrinus isolated from stimulated whole saliva samples of 91 caries-free children and 87 SECC children were subcultured, identified by polymerase chain reaction and genotyped by arbitrarily primed polymerase chain reaction. Polysaccharide synthesis ability, acidogenicity, aciduricity and the adherence ability of these S. sobrinus isolates were measured. Results: The frequency of S. sobrinus detection was 18.39% (16/87) in SECC children, which was significantly higher than that (3.30%, 3/91) in caries-free children. One to three different genotypes of S. sobrinus were detected in each SECC child. Only one genotype was colonised in each caries-free child. In SECC children, the production of water-insoluble glucan (WIG) was positively correlated with the ability of S. sobrinus adhering to a glass surface. Conclusion: The presence of S. sobrinus could be a risk factor for high caries activity in severe early childhood caries. The multi-genotypes could be related to different caries suceptibility. Water-insoluble glucan plays an important role in the adherence and accumulation of S. sobrinus on tooth surfaces. Keywords: Streptococcus sobrinus, genotypic diversity, virulence trait, cariogenic ability, severe early childhood caries