Plane strain fracture toughness (K/<IC>) measurements were made on ceramics (graphite, engineering ceramics) using short bar and bend bar chevron notched specimens previously calculated by optimum specimen geometry, to enable precracking, by means of 3-dimensional finite element and Bluhm's slice model analysis. Acoustic emission (AE) signals were also measured for detecting precracking and successive crack growth. AE signals due to precracking and crack growth were obtained for all short bar and bend bar specimens of graphite ; and deviations of K_IC values were relatively small with variations in chevron notch angle and the width of the slot from 0.2 mm to 0.5 mm. In most engineering ceramics, unstable fracture immediately occurred after popin. About 20% higher K_IC values were obtained in comparison to another testing device (fractometer-Terratek Co.), except for one commercial hot-pressed Si₃N₄. It means that the width of the slot should be less than 0.2 mm in highly brittle solids, and confirmation of precracking is actually important to evaluate K_IC value.