CO2 emission factors developed from fuel-specific data are more reliable than published average emission factors. In this article, CO2 emission factors were estimated for main combustion sources including gas turbines, boilers, and gas flares in an Iranian oil and gas processing plant using a fuel analysis method as well as direct measurements. In general, the emission factors for gas flares were higher than gas turbines and boilers. However, it is believed that CO2 emission factors are not very sensitive to combustion devices. CO2 emission factors varied significantly with the quality of fuels and operation conditions. The mean CO2 emission factor for a gas turbine was calculated as 114.47 lb/MMBtu (pound per million British thermal unit). This emission factor has good agreement with EPA and CAPP data. However, for the considered boiler due to poor fuel quality, an unexpected high mean value emission factor of 171.57 lb/MMBtu with uncertainty of 23.84% was obtained. The mean emission factor for gas flares was calculated as 122.15 lb/MMBtu and 112.18 lb/MMBtu with less than 1.5% uncertainty for combustion efficiency values of 98% and 90%, respectively. The results showed that among gas flares, the acid gas flares' CO2 emission factors are high and strongly dependent to gas composition and heating value.