The alumina-spinel ultra-low cement castables (ULCC) has been explored using tabular alumina as aggregates and calcined alumina and/or a novel spherical alumina fine as filler. The novel spherical alumina, labelled as AloxX-Spheres™ 99, consists of solid spheres with an average particle size distribution of 2-5µm. In this paper, the effect of morphology of alumina on mechanical properties, hot-properties, and thermal-shock resistance of alumina-spinel ULCC were evaluated in detail. To understand the fracture mechanism, the Wedge-Splitting Test (WST) was performed on notched prismatic specimens that enable stable crack formation for relatively large specimens. The fracture mechanical parameters such as the nominal notch tensile strength and specific fracture energy were calculated. The results confirmed that the specimens of the alumina-spinel ULCC with the introduction of spherical alumina not only increased the mechanical strength but also enhanced its thermal-shock resistance due to improved toughness. SEM photos of the specimens after fired at temperature at 1000°C showed that the addition of spherical alumina fines had strong impact on crack-propagation and worked as a “crack-stopper”. With the increment of spherical alumina content, both the crack length and the proportion of crack propagation in the grain decreased significantly.