Low noise AE waveform analyzing system, especially for ceramics, was established and microcracks nucleated during fracture toughness testing of Al2O3 were evaluated using this system. Three dimensional locations of microcracks were carried out using arrival time difference of 6 ch AE waveform signals. In the earlier stage, microcracks were nucleated around the center of notch front, and in the latter stage, their sites were widened through the specimen thickness. Most of these cracks were evaluated as mode I cracks because of the same phase radiation pattern of AE signals detected. Furthermore, the quantitative sizes of there microcracks were estimated as 15-50 μm by AE source characterization. Consequently, after being evaluated these microcracks were classified into two groups based on their sizes, i.e. smaller (15-30 μm) and larger (30-50 μm) cracks. It was concluded that the former were effective microcracks used to enhance the fracture resistance of ceramics, while the latter became the origin for the final unstable fracture.