Fine grain alumina and mullite particles (approximately 5 mu-m) were incorporated into slip-cast alumina - mullite refractories in order to investigate their effects on the microstructure, mechanical properties and thermal shock behaviour of the refractories. The incorporation of alumina particles improved both densification and mechanical properties markedly, compared to that of mullite particles, which resulted in high porosity and low mechanical properties. The crack initiation resistance increased with the addition of alumina, which is supported by R parameter. Therefore, more fracture surface energy was required to connect the cracks for propagation, associated with a high value of fracture toughness. The strength and Young's modulus were also improved with increasing quench temperature, leading to a high thermal shock resistance. (C) 2002 Elsevier Science B.V. All rights reserved.