The impact of changing energy mix on CO2 emissions consists of two aspects: Changing the fossil fuel-carbon factor (defined as CO2 emissions/fossil fuel) on CO2 emissions, and changing the share of fossil fuel in TPES on CO2 emissions. This study develops a method for analyzing the impact of changing the above two aspects on the formation Of CO2 emissions. The case analysis Of CO2 emissions in the OECD from 1971 to 2000 is used to demonstrate that the method can realize its designed purpose. The case analysis confirms that there was a huge decrease in actual CO2 emissions in the OECD from 1971 to 1995 when compared to the theoretical trend. Furthermore, it shows this occurred due to decreases in 1) the fossil fuel-carbon factor 2) the share of fossil fuel in TPES, and 3) energy intensity. However this situation changed from 1996 to 2000 due to the increase in the share of fossil fuel in TPES.