Mixed matrix membrane is a highly potential material that may possibly overcome the challenges faced by the membrane technology today. We explored the gas separation performance of a modified Matrimid/C-60 mixed matrix membranes. The membranes were prepared by physically blending a series of benzylamine-modified fullerene, C-60 with pure Matrimid 5218. The bulk Matrimid and the dispersed benzylamine-modified fullerene molecules show strong interfacial interactions as reflected by a significant increase in the T-g of the film with C-60 loading. Gas permeabilities of He, O-2, N-2, CH4, and CO2 show a monotonous decrease with increasing benzylamine-modified C-60 content. The percentage of permeation reduction at 10 wt.% benzylamine-modified C-60 loading is 32% for He (kinetic diameter of 2.6 Angstrom), while 41.7 and 41.5%, respectively, for the O-2 (3.46 Angstrom) and N-2 (3.64 Angstrom) gas pair, 45.2 and 47.0%, respectively, for CH4 (3.8 Angstrom) and CO2 (3.3 Angstrom) gas pair. The decrease of the gas permeability with increasing C-60 compositions is found to be mainly due to the decrease in the gas diffusivity. The presence of the benzylamine-modified C-60 seems to serve as impenetrable volumes and rigidifying elements within the polymer matrix. Therefore, the diffusivity of larger penetrants is reduced to a greater extent than the smaller penetrants and results in the increase of He/N-2 selectivity. This is also supported by the sorption data, which reveals that the Langmuir sorption capacity decreases with increasing C-60 loading. The increase in the glass transition temperature and the measured density indicate that the presence of the chemically modified C-60 molecules have rigidified the polymer and pulled the matrix together.