Group B Streptococcus (GBS) is one of the most important bacteria in the majority of maternal and neonatal infections, such as chorioamnionitis, endometritis, bacteremia, sepsis and meningitis. During pregnancy, GBS screening is one of the recommended strategies that are recommended by center of disease control (CDC). This study was aimed to determine the rectovaginal colonization prevalence among pregnant women, and also the rate of transmission to their offspring. Between June 2008 and April 2009, two hundred pregnant women admitted in department of Obstetrics and Gynecology (Ghaem Hospital, Mashhad, Iran) were enrolled in present study. Samples from maternal rectum and vagina as well as neonate ear and umbilical cord were taken for culture. The colonization rate for GBS in pregnant women and their neonates was around 6% and 5% respectively. All the carrier mothers were cases with premature rupture of membranes (at least 18 hours before delivery). In terms of colonization, there was a significant correlation between mothers and newborns, and more than 80% of neonates from GBS carrier mothers were colonized by GBS.