Poly(methyl methacrylate)/mesoporous molecular sieve (PMMA/MMS) composites were prepared by in situ polymerization. The structural feature and physical properties of these composites were studied by IR, DSC, SEM, DMTA, thermogravimetry JG), etc. The thermal stability, glass transition temperatures, tensile strength and Young's modulus of the composites increase with the increasing concentration of MMS. The reason of such improvement was examined and interpreted with a structural model named as "interpenetrating organic-inorganic network" (IOIN). Due to the interpenetrating of organic polymer chain with inorganic scaffold, the interface interaction between the surface of inorganic particles and the polymer resin was improved. As a trial, the composite possessing the structure of "interpenetrating organic-inorganic network" may be a hybrid material with improved physical properties. (c) 2007 Elsevier B.V. All rights reserved.