Abstract: Streptococcus suis is a swine pathogen which causes serious inflammation of piglets. It can infect people in contact with diseased pig or its byproducts. The mechanisms of S. suis pathogenesis are poorly understood. Taking two S. suis serotype 2 strains 606 isolated from China and 607 from Japan as samples, opacity factor of S. suis (ofs) genes were amplified from the genomic DNA of these bacteria by PCR method. The ofs genes contained 3 016 bp which encoded for 938 aa based on alignment analysis. The identity of nucleotide sequence of ofs gene from SS2 strain 607 was 100% to the reported SS2 genes while seven nucleotides in ofs of SS2 strain 606 were different and resulted in three amino acids substitution. One of the substitutions located at the Nterminal region of OFS protein in which glycin (without charge) was changed to be aspartic acid (with negative charge). It was proposed that the change would have an effect on the OFS ability to opacify serum of mammals. The other two substitutions were found from the repeats in Cterminal region of OFS in SS2 strain 606. Furthermore, the length and amino acid constitute of three repeats in OFS of SS2 strain 606 and 607 was very divergent from that of fibronectinbinding protein A (FnBA) of Streptococcus dysgalactiae. It was suggested that the pathway of OFS protein binding to fibronectin of host cells might be different from that of FnBA. However, OFS proteins did carry the typical structural elements of MSCRAMMs (microbial surface components recognizing adhesive matrix molecules) with a large Nterminal region, specific repeats and LPXTG domain elements in the Cterminal region. It could be presumed that OFS of SS2 strain 606 and 607 function as an adhesin combined on the surface of bacteria and bind the fibronectin with its repeats at the Cterminus. The correlation between the OFS with moderate size (938 aa) and the high virulence of streptococci implied that ofs gene was a pivotal virulent gene of SS2 strain 606 and 607.