:The cause of infertility is either fertilization failure or early embryonic death. The aetiology may involve a combination of many factors, e.g. genetic factors, abnormalities in the gametes nutritional disorders, inadequate luteal function, and delayed ovulation. This study was conducted to investigate the effect of microorganisms collected from uterus of Hanwoo cattle on early embryonic development. Microorganisms isolated from the uterus of Hanwoo cattle included Bacillus cereus (Bc), Bacillus thuringiensis (Bt), Staphylococcus warneri (Sw) and Enterococcus faecalis (Ef).When cultured with Bc, Bt, Sw, and Ef, oocytes were not developed into a blastocyst in vitro. The proportion of blastocyst was dramatically increased after reducing the number of microorganisms (10 4 CFU/ml). Interestingly, the proportion of blastocyst was decreased by adding the Sw and Ef. These results indicate that among intrauterine microorganisms, Sw and Ef strains negatively affect theearly embryonic development, thereby aggravate the rates of implantation and pregnancy. These findings will provide useful information for association studies in other pig populations.