Horseradish peroxidase (HRP) was immobilized in a mesoporous material (folded sheets mesoporous materials, FSM-16) and then entrapped in organic/inorganic hybrid gel comprising various molar ratios of dimethyldimethoxysilane (DMDMOS)/tetramethoxysilane (TMOS). When pore size of FSM-16 materials is much larger than the diameters of horseradish peroxidase (HRP), the residual enzymatic activity after thermal treatment (70 degreesC, 60 min) increased from 73 to 99%, and the oxidative conversion yield of 1,2-diaminobenzene in an organic solvent increased from 59 to 79% after 4 h and the level of leakage of immobilized HRP decreased from 6 to 1.5% on washing by secondary hybrid gel entrapment comprising a molar ratio of DMDMOS/TMOS=1:3. When pore size of FSM-16 materials just matches the diameter of the enzyme, the conversion yield in an organic solvent and the level of leakage of immobilized HRP did not change so much.