The purpose of this paper is to investigate the influence of grain size and temperature on fracture toughness of MgO sintered bodies, Specimens were made by hot pressing, and a wide range of grain size (12-230μm) was obtained by annealing at high temperatures. Porosities of specimens were less than 1%. Fracture toughness of MgO sintered bodies with different grain size, was measured by the Chevron-Notched Beam method at room temperature, 1000° and 1200°C. KIC did not depend on grain size at room temperature, and took the constant value of 1.8MPa√m over a wide range of grain size. On the other hand, KC increased with decreasing grain size at 1000° and 1200°C. This increase in KC at small grain size corresponded to the macroscopic ability of plastic deformation in bending test. Equivalent crack size at room temperature was calculated. Equivalent crack size slightly increased with increasing grain size, and subsequently saturated. This tendency suggested that grain boundaries controlled the growth of microcrack at small grain size.
