The molecular structures of methacryl-grafted polysiloxane resins from 3-(trimethoxysilyl)propyl methacrylate (MPTS) and diphenylsilanediol (DPSD) were determined by theoretical computation and experimental measurement. The molecular structures obtained from theoretical computation coincided well with those from experimental measurement, and we found that the structural changes in the resins could be controlled by precursor compositions. While molecular weights and polysiloxane chain lengths of the resins increase with DPSD contents, their molecular sizes do not vary significantly. In the present study, molecular sizes and shapes of the resins with various compositions are hypothesized theoretically and proven experimentally. (c) 2005 Wiley Periodicals, Inc.