Hydroxypropyl cellulose (HPC) was incorporated in situ with silica by the sol-gel process involving various amounts of added tetraethoxysilane (TEOS), and hybrids with nanometre-sized silica particles were prepared. To determine the effect of TEOS content on the mechanical properties of the micro-hybrids, the TEOS content in the hybrids was varied from 20 to 60%. The mechanical properties of the micro-hybrids were compared with those of HPC composites mixed with various amounts of glass beads (diameters of 20-40 mu m) by solvent blending. The ultimate tensile strength of the micro-hybrid was improved from 6 to 18 MPa when the TEOS content reached 40 wt%. On the other hand, the strength of the blend composite decreased with glass-bead content. The alpha(1)-(molecular motion in a highly ordered amorphous phase), alpha(2)-(molecular motion in a random amorphous phase) and beta-relaxations (relating to molecular motion of hydroxypropyl groups) were observed in the temperature dependence of the dynamic viscoelastic properties of HPC. The level of the dynamic modulus, G(’), of the micro-hybrid increased with initial TEOS content. By hybridization, the alpha(1)-peak in the tan delta curve broadened and its intensity lowered as the initial TEOS content increased. The alpha(2)-peak was observed for the micro-hybrid composed of 20 wt% TEOS, but disappeared when the TEOS content exceeded 40 wt%. The beta-peak was not observed in the hybrid. No drastic change was observed in the dynamic viscoelastic behaviour of the blend composite as the glass-bead content was varied. The mechanical properties of the in situ micro-hybrid were completely different from those of the blend composite. This difference was caused by the interaction between the filler and matrix. In the micro-hybrid, nanometre-sized ceramic particles may be dispersed in the HPC component, and strong interaction occurs via hydrogen bonding. In contrast, the interaction between the glass beads and HPC in the blend composite is weak.