The effect of filler silica particle size on the flexural strength and the fracture toughness (K_IC) of silica particle-filled epoxy composite was investigated. As the particle size increased the flexural strength decreased and the K_IC increased. The large filler acts as the defect which causes fracture initiation for the flexural strength. On the other hand, the stress relaxation by the branching, the making a detour and the debonding of interface caused by the large filler developed the K_IC. In order to raise both strength and K_IC, the following interface of filler/resin considered to be ideal: the interface having the adhesion strength which is sufficient for the required mechanical strength of composite, whereas when the applied stress exceeds a threshold value, the debonding of the interface occurs and causes the stress relaxation.