In industrial electric production, cogeneration and fossil fuel power plants occupy a high proportion, resulting in excessive carbon dioxide emissions in the atmosphere. As a result, environmental issues such as global warming have emerged as major problems of our society. Various studies have been conducted in order to solve this problem. Among them, a method of utilizing magnesium oxide (MgO) as an adsorbent to capture carbon dioxide has been extensively studied. MgO has several advantages such as excellent theoretical CO2 adsorption capacity, thermally stable MgCO3 form, and low regenerative energy. However, MgO has also disadvantages such as slow reaction kinetic and loss of basic site on MgO surface during regeneration, large lattice energy and irregular particle size. Thus it is difficult to exert excellent performance in capturing CO2 with MgO alone. Recently, it has been found that the promoter can utilize MgO toward CO2 adsorption by dissociating it into Mg2 + and O2-. In addition, hydrotalcite as a support can function as a framework for forming MgCO3 with high stability, CO2 capture ability, and economic efficiency, thereby further enhancing the CO2 adsorption ability of MgO. Fundamentally, CO2 capture on MgO begins with surface chemisorption and ends up with forming MgCO3 throughout the MgO body. Adsorption-mediated carbonation can be promoted due to the strong solvation effect when the promoter and support are mixed. However, the CO2 adsorption mechanism on neither pure MgO nor the MgO complex of promoter and support has not been explicitly described for the formation of surface specific MgCO3 (on pure MgO) and bulk MgCO3 (on MgO with promoter and support). In this study, adsorbents were prepared by using promoter (NaNO3) and hydrotalcite (MgO: Al2O3, the weight ratio of 30:70 and 70:30) as a promoter for dispersion of MgO and stability of adsorbent. Adsorption performance of hydrotalcite used as a support was investigated by non-isothermal test according to weight ratio of MgO: Al2O3. Adsorbent characteristics were analyzed by XRD, cyclic test, fixed bed adsorption test, and TGA. Also, adsorbents were subjected to in-situ transmission electron microscopy (TEM) for the direct observation of fundamental CO2 adsorption in a complex of MgO, promoter and support under non-vacuum (CO2 atmosphere) and high temperature (> 250oC) conditions. Morphological and crystallographical changes at the interface of synthesized mixture were observed in nano-scale by using SAED and EDS. The performance of the adsorbent and the surface change of complex by analysis of CO2 adsorption mechanism will be discussed in detail. This work was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRT-2016R1C1B2008694).