Abstract: The Group B Streptococcus (GBS) is one of the most important cause of neonatal Infection and, there is some evidence that it has an important role in vaginal infection with PROM, preterm labor, chorioamnionitis, pyelonephritis, metritis and postpartum infection; many researcher in all around the world try to declare true relation of GBS vaginal colonization with above mentioned complications. If this relation can be proved, its treatment can eliminate related complication. In this study, we searched the rate of GBS colonization in pregnant women that were admitted in Alzahra Hospital and searched for the probable cause and risk factors, correlation of them with postpartum complications with the hope that this study become the first step to show the way for prevention, diagnosis and treatment. In this study, we got two swabs from vagina and rectum of 250 pregnant women that were admitted to labor in Alzahra Hospital. The inclusion criteria was third trimester pregnancy, not used any antibiotics last 72 h before admission and not have any vaginal or rectal exams before taking the sample. Swabs were inoculated in 2 media Blood-agar and Todd Hewitt, then the media transfer to microbiological lab and searched for GBS. All demographic information was collected by a questionnaire from every case and the patients were following up until one month after delivery. In this study the prevalence of positive vaginal GBS culture in pregnant woman was (9.6%). We did not find any significant correlation between this positive culture with gestational age, family income level, post partum infection, neonatal infection and the indication of admission. Correlation with positive rectal culture, age of woman and use of IUD before pregnancy was significant. The most prevalent age of positive vaginal GBS culture was under the 25 year old. In this study significant correlation between vaginal and rectal colonization shows gastrointestinal origin of this organism due to ascending of infection from rectum to vagina. That can be prevented by means of health preventative methods. From contraception methods, the women that had used IUD had more GBS (significant correlation with positive culture of vaginal group B streptococcus) (p= 0.004). Correlation between age of women and positive culture of group B streptococcus was significant p = 0.0001.