Thermal shock tests were performed on hot pressed AlN and SiC using hydrogen beams as an energy source to heat large samples rapidly. The single-shot-resistance to thermal shock spalling was examined under three combinations of heat flux density and pulse duration time: 8.8. kW/cm2 - 56.7 ms, 2.5 kW/cm2 - 100 ms, and 4.7 kW/cm2 - 100 ms, respectively. SiC survived a thermal shock with 2.5 kW/cm2 - 100 ms, but failed under the other two heating conditions. AlN was spalled even by a thermal shock with 2.5 kW/cm2 - 100 ms. The experimental observations on the tests with a thermal shock of 100 msec imply that the thermal shock cracking has occurred in the heating process rather than in the cooling process and that the cracks have not been initiated on the heated surface.