This study investigated the degradation in mechanical properties of an alumina subjected to thermal shock loading by systematic experiments on bend-bar specimens of two sizes, 3 x 4 x 45 mm and 9 x 12 x 15 mm, Thermal loading was applied by water-quench method with several well defined temperature differences. Critical temperature difference (Delta T-c) was defined as the lowest applied temperature difference above which a loss in flexure strength occurred. Delta T-c values for the large and small specimens were found to be 175 and 200 degrees C, respectively. The reduction in thermal shock resistance with increasing sample size can be well described by the thermal elastic and fracture energy theories. However, if the size effect on the initial, unshocked fracture strength is considered, the size effect on the transport of heat during quench test might play a less important role in reducing the thermal shock resistance of large specimen.