The authors determined the prevalence of variant genes of Shiga toxin gene (stx1, stx1c, stx2, stx2c, stx2d, stx2g, stx2) of 34 E. coli O157:H7 strains which were isolated from healthy cattle and buffaloes in some provinces in the central region of Vietnam. The results showed that 27/34 strains (79.41 percent) carried genes for stx1 toxin, 29/34 strains (85.35 percent) carried genes for stx2 and 22/34 strains (64,71 percent) carried both of two genes for stx1 and stx2 toxin. Further, characterization of stx subtypes demonstrated that among 27 strains carried genes for stx1, 18 strains (66.67 percent) carried stx1c gene and 9 strains (33.33 percent) carried stx1. Of the 29 strains carried genes for stx2, 5 strains (14.71 percent) carried variant gene stx2c, 9 strains (31.03 percent) carried variant gene stx2d, 15 strains (55.72 percent) were stx2 and 3 strains (6.90 percent) carried stx2g gene.