Lanthanide hexaaluminates including LaMgAl 11 O 19 , NdMgAl 11 O 19 , SmMgAl 11 O 19 and GdMgAl 11 O 19 were synthesized via Sol–Gel method. Due to the anisotropic crystal growth, these oxides crystallize in the form of platelets and the platelet thickness increases with the decrease of rare-earth ionic radius. It was observed that the thermal-shock resistances of LaMgAl 11 O 19 , NdMgAl 11 O 19 and SmMgAl 11 O 19 oxides were superior to 8YSZ as proved by water quenching tests. In addition, the thinner the platelet, the more interstices are retained in the sintered specimen, and the better thermal-shock resistance the oxide has. Based on SEM images, it can be seen that the SmMgAl 11 O 19 sample exhibits a mixture of the intergranular and transgranular fracture after thermal cycling failure.