Preterm birth and associated conditions are one of the biggest killers of children in the worldwide. In this regards, current study aimed at investigation of the relationship between air pollutants of Pm 10 , CO, O 3 , and SO 2 and preterm birth occurrences recorded in Imam Reza (PUH) Hospital of Mashhad for a five-year period from 2007 to 2011. The results of quantile regression showed that the amount of atmospheric pollutants have an important role in rising preterm births. In this way, the pollutants of Pm 10 and CO have revealed the strongest effect on preterm birth occurrences especially for the last three months of pregnancy. Moreover, preterm births mostly have occurred in warm seasons of spring and summer, while the seasonal average of pollutants concentration, excepting O 3 pollutant, has shown rising in cold seasons of autumn and winter. However, reducing air pollution could also be effective in reducing preterm births across the world.