Utilization of CO2 (carbon dioxide) to synthesize useful organic compounds has attracted much attentions from the academia and industry. Recently, the fixation of CO2 catalyzed by using four hydroxyl-functionalized ionic liquids (ILs) has been successfully achieved. However, the detailed mechanism is still ambiguous. We performed a theoretical study on the mechanism of the fixation of CO2 catalyzed by 2-hydroxyl-ethyl-triethylammonium bromide (HETEAB) using the DFT method. The structural and energetic information for each possible route were obtained. Furthermore, the similarities and differences of two different catalysts, HETEAB and 2-hydroxyl-ethyl-tributylammonium bromide (HETBAB), were discussed in detail. In addition, the result shows that the OH functional group is one essential factor to improve the catalyst ability. Except that, including active H atom in the IL is the other major factor to refine the catalytic performance. (C) 2015 Elsevier Ltd. All rights reserved.