Polyamidoamine (PAMAM) dendrimers were loaded in mesoporous silica MCM-41, and its gas adsorption properties were evaluated for a N-2, H2O, and CO2 to examine whether it could be used for CO2 separation. The mesoporous structure of MCM-41 was affected by the amount of dendrimers loaded. The behavior of N-2 and water vapor adsorption changed significantly with the PAMAM loading rate owing to the plugging of the MCM-41 pores by the excess amount of PAMAM dendrimers. In dry conditions, although the CO2 adsorption capacity at high CO2 partial pressures gradually decreased with an increase in PAMAM loading, MCM-41 loaded with PAMAM at a high loading rate showed slightly larger CO2 adsorption capacity at low CO2 partial pressures, which resulted from the chemical adsorption of CO2 by amines. In the presence of water, neat MCM-41 showed a significant decrease in its CO2 adsorption capacity similar to the case of MCM-41 loaded with PAMAM at a low loading rate. On the other hand, for a high loading rate, the adsorbent showed much higher CO2 adsorption capacity than the neat MCM-41. Water condensation in the pores must lead to a significant decrease in the CO2 adsorption capacity. The CO2 adsorption capacity could be determined from the pore structure after the modification as well as from the nature of the modified amines.