Indoor air quality in animal barns directly affect to animal productivity. Measuring the pollutants in animal barns proves the negative effects of gases on health of animals and workers. Most studies in this context focused on some pollutant gases such as ammonia, carbon dioxide, methane and hydrogen sulfide in dairy barn and poultry houses. Less attention in studies in scientific literature was paid to sheep barns which may have more important portion than other animals for the animal production sector of some countries. This paper revealed the concentrations of ammonia (NH3) and carbon dioxide (CO2) in naturally ventilated sheep barn in Bursa region, western Turkey. Also indoor environmental conditions such as temperature and relative humidity were measured simultaneously with pollutant gas concentrations. Regression and variance analyzes were applied to assess data collected in sheep barn whole over the study period. The average NH3 concentration during the study was 15 ppm for exhaust and 0.77 ppm for ambient, CO2 concentration was 1022 ppm for exhaust and 457 ppm for ambient. There was significant difference among measurement days for exhaust NH3 and CO2 concentrations (P< 0.001). The lowest NH3 concentration was 8 ppm while lowest CO2 concentration was 277 ppm, and the highest concentrations were 38 ppm for NH3 and 1700 ppm for CO2. Also minimum, maximum, average values for indoor temperature were 16.06 degrees C, 26.53 degrees C, 20.69 degrees C, respectively, while minimum 43.42%, maximum 89.6%, average 71.23% values for relative humidity were obtained. According to regression analyze results, the exhaust NH3 concentration related statistically significant with air velocity. The standardized coefficients suggest that indoor temperature and relative humidity sustained the greater effects on CO2 concentrations (P< 0.001).