Electricity is the basic need of most economic sectors within a national economy. Electricity generation not only directly affects the amount of CO2 emissions, but it also indirectly affects a country’s economic system. For Thailand, the electricity generation sector represents the largest source of CO2 emissions, so it is necessary to investigate the potential factors contributing to the changes in CO2 emissions from this power sector. Here, a decoupling method was used to evaluate the relationships between energy consumption and the CO2 emissions from Thailand’s thermal power generation that were caused by economic developments during 2000–2011. Key factors affecting the evolution of CO2 emissions from Thailand’s thermal power sector were analyzed by Divisia index decomposition. Changes in the emission coefficient, heat rate, fuel intensity, electricity intensity and economic growth were investigated. The results reveal that energy consumption and CO2 emissions were coupled during 2000–2005, whereas a relative decoupling appeared for 2006–2011. Moreover, the economic effect was the critical factor for increased CO2 emissions from Thailand’s thermal power generation, while electricity intensity played a dominant role in decreased CO2 emissions. Since the CO2 emissions released from Thailand’s electricity generation are rapidly increasing, the Thai government will be required to reduce CO2 emissions in the future by enhancing energy conservation, reconstructing the fuel mix in power generation, promoting a shift in the economic structure toward less energy-intensive services, and orienting Thailand’s power industry towards low carbon electricity generation.