We analytically present the characteristic dimensional limit below which the thermal shock failure of ceramics never occurs. This limit, together with the critical temperature difference, separates the state space of the ceramics under thermal shock into two parts -the cracked and the uncracked. Based on the water-quench tests of ceramics, we experimentally proved that when the states of ceramics are in the uncracked region, the ceramics do not produce any cracks during thermal shock. The results provide a guide to prevent thermal shock failure in ceramic.Keywords: thermomechanical; ceramics; fracture; thermal shock 1. Introduction Thermal shock failure of ceramics widely occurs in the thermostructural applications of ceramics. More than one-third of the rejections of ceramic components are caused by thermal shock [1]. Previous studies point out that besides the properties of materials, two external factors of ceramics, the characteristic dimension and the temperature difference, play a key role in the thermal shock failures of ceramics [2][3][4][5][6][7]. For example, the thermal shock resistance of ceramics increases with the decrease of characteristic dimensions [2][3][4], and ceramics are prone to thermal shock failures at higher temperature differences [2][3][4]7]. However, the quantitative effects of the two factors on the thermal shock failures of ceramics have not been understood very well [8]. In this study, firstly, we obtained a relationship between the two critical external factors of ceramics and thermal shock failures based on the theories of heat transfer and thermal stresses. Then, from the water quench tests of alumina, the relationship presented here proved to be in good agreement with the experimental results.