Silica mesoporous materials were synthesized and used as the supporting materials for immobilization of enzyme. Penicillin G acylase, an enzyme which was used to produce 6-APA in pharmaceutical industry, was immobilized in the mesoporous materials by the immersion method. The stabilities of the immobilized penicillin G acylase were studied. After incubation at 60°C for 2 h, the activity of immobilized penicillin G acylase remained 80% in the best case. At higher or lower pH, the free enzyme was deactivated quickly, while the immobilized enzyme still retained active. The result of operational stability showed that the immobilized enzyme retained 70% of its initial activity after operating for 6 times. These results showed that the stabilities of immobilized penicillin G acylase, related to the pore size of mesoporous materials, were increased significantly compared with those of free enzyme. The improvement of stabilities of immobilized enzyme was significant when the pore size of the mesoporous materials matched the enzyme molecule size.