The MCM-41 and Co-MCM-41 molecular sieves were confirmed by characterization with XRD, FTIR, nitrogen adsorption, and NH3-TPD. Denitrogenation of model fuel containing about 1,737.35 g (nitrogen)/g or diesel oil was studied over the samples. The molecular size of quinoline, calculated by using density functional theory, was 7.116 x 5.002 angstrom, implying that it easy access to mesoscale pores of molecular sieves. The adsorption denitrogenation capacity of Co-MCM-41 was clearly more effective than MCM-41 due to its strong acidity and chemisorption. The basic nitrogen removal of diesel oil on MCM-41 or Co-MCM-41 was 68.22 and 65.91%, respectively.