During recent years, air pollutants were important social issues in the Republic of Korea. Air pollution is a major global health concern and effects on many health problems. In this study, we analyzed association of adverse birth outcomes (preterm births, low birthweights) and ambient air pollutants. A total of 1,802,925 records of singleton births were examined using the National Birth Registration Database, from 2010 to 2013. We obtained concentration of sulphur dioxide (SO₂), nitrogen dioxide (NO₂), carbon monoxide (CO), ozone (O₃), and particulate matter less than 10 micrometers in diameter ( PM10) from 2009 to 2013. Mean concentration of each air pollutants during pregnancy were calculated and matched to birth data by registered regions. We analyzed proportion of adverse birth outcomes between two groups by dividing air pollutant concentrations with World Health Organization criteria. Add to that, we compared the proportions between the first quartile and the rest of concentrations. A multivariate logistic regression model was used to assess the effect of air pollution in each region on preterm birth (less than 37weeks gestation) and low birthweight (less than 2,500 gram). The adjusted odds ratio (aOR) of PM10 over 70㎍/m³ for preterm birth was 1.530 (C.I. 1.436 – 1.630). The aOR of highest quartile of SO₂ was 1.056 (C.I. 1.046 – 1.067) and CO (aOR: 1.031, C.I. 1.020 – 1.041), O₃ (aOR: 1.139, C.I. 1.127 – 1.152), and PM10 (aOR: 1.022, C.I. 1.012 – 1.033) were associated with preterm births. However, low birthweight was not associated with exposure to air pollutants. Maternal exposure to ambient air pollutants during pregnancy was associated with preterm birth. Especially, expose to PM10 over 70㎍/m³ may increase the risk of preterm birth. Further research for the mechanism and education for pregnant women to avoid exposure are needed.