Four-point bending tests were carried out for silica-particle-filled epoxy composites. Resin-rich (Type R) and cut (Type C) surfaces were taken as the tensile surfaces in order to investigate the effect of volume fraction of silica particle on microfracture process. AE measurement was performed during the test using the two AE sensors attached to both the ends of the specimen. Rapid increasing point of cumulative AE energy was observed before final fracture in both specimens. AE source location at this point corresponded to the location of final fracture. It was then suggested that the maincrack is formed at this point. The critical stress for maincrack formation and the final fracture strength were defined as σ_C and σ_B, respectively. The average value of σ_C of Type R was higher than of Type C. It was suggested that the maincrack is formed from silica particle. Furthermore, the average value of σ_B-σ_C of Type C was higher than of Type R. It was suggested that crack growth resistance increases with increasing volume fraction. Consequently, the critical stress, σ_C, would be available as the conservative design parameter than the final fracture strength.