Abstract: In this study the modified chitosan membranes were prepared by dispersing and removing the ultrafine SiO₂ particles (5-15 μm) and observed with SEM. The analysis showed that the modified chitosan membrane had an asymmetric structure which was composed of micro-scale porous structure and smooth surface structure. The mechanical properties of the chitosan membrane before and after the modification had not changed significantly. The results of pure water permeability used a dead-end filtration experiment showed that the water permeability of the modified chitosan membrane was 1.5 times higher than that of the unmodified because the micro-scale porous structure could increase the effective diffusion area of the membrane. Experiment results using free diffusion method to test the effective diffusion coefficients (D_eff) of lysozyme (14 kDa), trypsin (20 kDa), pepsin (34 kDa), chicken ovalbumin (43 kDa) and bovine serum albumin (65 kDa) on the chitosan membrane showed that the D_eff of the modified chitosan membrane varied as high as 300 time, though the molecular weights of proteins were changed only 4.6-time. Thus, the results indicated that the modified chitosan membrane had performed a certain protein selective filtration. On the other hand, the effective diffusion coefficients of ovalbumin and bovine serum albumin have changed significantly. It was speculated that the size of the mass transfer channel of the modified chitosan membrane was approximately equivalent to the molecular size of ovalbumin. This paper provided a basic study on bio-enzyme refinery process by membrane technology.