Abstract: The cracking mechanism and crack distribution law of thermally shocked ceramics were studied. The water quenching experiment of 99Al2O3 ceramic specimens with thickness of 1mm showed that the spacing of cracks decreased with the increase of thermal shock temperature differences. The difference between individual crack spacing and the average spacing didn’t exceed 7% in 5 specimens at the same thermally shocked temperature difference. The transient temperature field and stress field were calculated by combining heat transfer and mechanical theory, and the cracking mechanism of ceramics under thermal shock was studied. A finite element method for predicting thermal shock crack spacing was developed by using the minimum energy principle with the crack spacing and depth as variables. In view of large dispersity of the Biot number in references and its difficulty in direct measure, an “indirect measure method” was put forward, the Biot number in experiment was inversely estimated, and the numerical predictions at other temperature differences which agreed well with experimental results were obtained. The present research provides help for deeply understanding the failure mechanism of ceramics under thermal shock, improving the performance and designing new ceramics.