Although aqueous ammonia solution has been focused on the removal of CO2 from flue gas, there have been very few reports regarding the underlying analysis of the reaction between CO2 and NH3. In this work, we explored the reaction of CO2-NH3-H2O system at various operating temperatures: 40℃ , 20℃ , and 5℃ . The CO2 removal efficiency and the loss of ammonia were influenced by the operating temperatures. Also, infrared spectroscopy measurement was used in order to understand the formation mechanism of ion species in absorbent, such as NH2COO6(-), HCO3^(-), CO32^(-), and NH4+, during CO2, NH3, and H2O reaction. The reactions of CO2-NH3-H2O system at 20 ℃ and 40 ℃ have similar reaction routes. However, a different reaction route was observed at 5 ℃ compared to the other operating temperatures, showing the solid products of ammonium bicarbonates, relatively. The CO2 removal efficiency and the formation of carbamate and bicarbonate were strongly influenced by the operating temperatures. In particular, the analysis of the formation carbamate and bicarbonate by infrared spectroscopy measurement provides useful information on the reaction mechanism of CO2 in an aqueous ammonia solution.