Effects of immobilization pH and pore characters of mesoporous silicas (MPSs), MCM-41, SBA-15, and MCF, were simultaneously investigated for the immobilization of horseradish peroxidase (HRP; EC 1.11.1.7). MCM-41 and SBA-15 were rod-like with respective average pore diameters of 32, and 54 Å, while that of MCF with spherical cell and frame structure was 148 Å. Moreover, the MPSs synthesized were of identical surface functional groups and similar contents of free silanol groups. At immobilization pH 6 and 8 almost 100% HRP loadings were obtained and insignificant leaching were observed for all types of supports at pH 6. However, MCF was found to give both the highest enzyme loading and leaching at pH 10. Maximum and minimum HRP activities were obtained at respective immobilization pH 8, and 6. Activities of immobilized HRP increased with support pore diameters in the order: MCM-41 < SBA-15 < MCF. HRP immobilized at pH 8 gave the highest storage stability (both at 4 °C and room temperature), and in opposition to pH 6. In addition, HRP immobilized in MCF was found to be the most stable under storage. The finding should be useful for the creation of biocatalysts and biosensors.