Adsorption of pure CO and N and separation of CO /N mixture in MFI zeolite and MFI/MCM-41 micro/mesoporous composite have been studied by using atomistic simulations. Fully atomistic models of MFI and MFI/MCM-41 are constructed and characterized. A bimodal pore size distribution is observed in MFI/MCM-41 from simulated small- and broad-angle X-ray diffraction patterns. The density of MFI/MCM-41 is lower than MFI, while its free volume and specific surface area are greater than MFI due to the presence of mesopores. CO is preferentially adsorbed than N , and thus, the loading and isosteric heat of CO are greater than N in both MFI and MFI/MCM-41. CO isotherm in MFI/MCM-41 is similar to that in MFI at low pressures, but resembles that in MCM-41 at high pressures. N shows similar amount of loading in MFI, MCM-41 and MFI/MCM-41. The selectivity of CO over N in the three adsorbents decreases in the order of MFI>MFI/MCM-41>MCM-41. With increasing pressure, the selectivity increases in MFI and MFI/MCM-41, but decreases in MCM-41. The self-diffusivity of CO and N in MFI decreases as loading increases, while in MFI/MCM-41, it first increases and then drops.