Adsorption separation Of CO2 from simulated flue gas containing CO2, O-2, and N-2 with and without moisture was investigated using a novel nanoporous adsorbent based on polyethylenimine (PEI)-modified mesoporous molecular sieve MCM-41 in a flow system. The CO2 adsorption capacity and CO2 separation selectivity of MCM-41 were greatly improved by loading PEI into its nanosized pore channels, which made the resulting adsorbent operating like a "molecular basket" for CO2. CO2 adsorption capacity of the MCM-41-PEI adsorbent for the simulated moist flue gas was higher than that for the simulated dry flue gas. CO2 separation selectivity of the MCM-41-PEI adsorbent was also improved in the presence of moisture when compared with those in the dry gas condition. The influence of moisture concentrations in the simulated flue gas on the Co-2 adsorption separation performance was also examined. The results of adsorption/desorption separation cycles showed that the MCM-41-PEI adsorbent was stable over 10 cycles of adsorption/desorption operations. The hydrothermal stability of the "molecular basket" adsorbent was better than that of the MCM-41 support alone.