Escherichia coli are known as the most common cause of reproductive tract infection in mare. Due to the progressive process of antibiotics use and increasing prevalence of antibiotic resistance, the aim of this study is evaluate the prevalence of subclinical endometritis and antibiotic resistance genes in Escherichia coli isolated. In this study, 60 mares were used with infertility background. Diagnosis of endometritis was performed using history and ultrasonography. Cytology, culture, Antibiogram were done of samples and PCR test was used to examine the gene virulence and antibiotic resistance. E-coli bacteria was isolated 48/33 % from sample culture. In PCR test 66/21 % of bacteria had virulence gene. It was determined, the lowest resistance to chloramphenicol about 38/15% and greatest resistance into ampicillin, tetracycline and streptomycin with 23/69 percent, respectively. 93% samples cytology had neutrophil more than two and the agent of 50% showed E. coli. The cause of half of subclinical endometritis in infertile maresis E-coli bacteria.