It has been known that exposure to air pollutant during pregnancy was associated with low birth weight. To correlate NO2 concentration in ambient air with baby with low birth weight (LBW), an ecological study has been carried in Jakarta. NO2 concentration was obtained from 2009 – 2011 monitoring data (Jakarta BPLHD), while low birth weight data were obtained from Jakarta Provincial Health Office. Anova, correlation, linear and multiple linear regressions were employed to analyze NO2 concentration with LBW. It showed that NO2 concentrations during first and second month of pregnancy were significantly correlated with the LBW (R = 0.464, p value = 0.0001 and R = 0.243, p value = 0.013). Multiple linear regression showed that the concentration of NO2 in the first and second month of pregnancy can predict 25% of LBW cases (R = 0.5, R2 = 0.25; p value = 0.0001). The most influence variable on LBW is exposure to NO2 in the first month of gestation (B = 259). It is concluded that exposure to NO2 in the first and second month of pregnancy and city of residence correlated with the LBW, with NO2 exposure in the first month of pregnancy was the most influencing factor of the LBW.