OBJECTIVES: There is an increasing trend of stillbirth in Mongolia for the last few years and Ulaanbaatar city is the 2nd most polluted city in the world by WHO. This study sought to determine the association of outdoor air pollution and risk of stillbirth. METHODS: This was a case-control study. The case group consisted of 909 stillbirths and the controls included 159 764 live births from singletons born in 2008-2013 in Ulaanbaatar.  Air quality data between January 2007 and December 2013 was obtained. Association between air pollution and stillbirth was determined using Logit regression. Odds ratios were estimated per 10-ppb change for nitrogen dioxide (NO), sulfur dioxide (SO) and for particulate matter with aerodynamic diameter ≤ 10 μm (PM10), and 100-ppb change for carbon monoxide (CO) during different gestational periods. RESULTS: Average duration of gestation for stillbirths were 24.7 weeks whereas for live births average duration was 38.5 weeks (р<0.0001). 28.2% of stillbirths occurred in spring. Stillbirth risk increased in association with 10-ppb change for SO in all trimesters (OR=1.01; CI 95% 0.99-1.03). Stillbirth risk also increased in association with 10-ppb change for NO in first-trimester (OR=1.01; CI 95% 0.96-1.07). 100-ppb change for CO during all trimesters of gestation increased risks of stillbirth (OR=1.01; CI 95% 0.98-1.04). There was an increased risk of stillbirths in association with 10-ppb change for PMin first-trimester (OR=1.02; CI 95% 0.99-1.04). CONCLUSIONS: Exposure to outdoor air substances may increase the risk of stillbirth, and that the most susceptible time periods for exposure are during the first trimester of gestation.