The thermal shock behaviour of pure alumina and an alumina sialon composite was examined. The critical temperature difference of the ther.mal shock, DELTAT(C), had a good relationship with the calculated thermal shock resistance parameter, R. The thermal shock resistance of the composite was remarkably improved by simple modification, such as oxidation at 1400-degrees-C for 100 h. The oxide layer, with porous microstructure, consisted mainly of mullite and a small amount of alumina. The DELTAT(C) of pure alumina, the composite and the oxidized composite was 210, 225 and 360-degrees-C, respectively. This improvement was considered to be due to the characteristics of the oxide layer, which had high strength as well as a low elastic constant and a low thermal expansion coefficient.