A novel organic composite filler was prepared by the redox polymerization (BPO-amine system) of dimethacrylates in an ethanol-water solution in the presence of a colloidal silica filler (MF) , and the mechanical properties of the composite resin which contained the novel filler were examined. The homogeneous polymerization solution became heterogeneous up on the addition of water, and the monomers formed particles the surface of which was covered with MF particles and then polymerization occurred. The obtained particles were micorcapsule particles (MCP) containing 15-90wt% MF and having a diameter of 0.6-13μm.Composite resins containing the MCP or the mixed filler of MF and MCP were prepared, and their mechanical propertieswere investigated. The transverse strength of the composite resin containing 55wt% of MCP (MF content:15-80wt%) was about 100MPa. The transverse strength was lower than 100MPa, when the content of MCP (MF content:80wt%) was over 50vol%. The composite resin containing the mixed filler of MF and MCP, showed the maximum mechanical properties when it contained 70wt% mixed filler the MF content of which was 40-75vol%. Thus, the transverse strength was above 120MPa, the maximum bending attained the maximum value, the elastic modulus was around 7000MPa, and Rockwell hardness(30W)was above 60.