Effects of specific surface area and tiny amount of impurities of reactive alumina on the workability, sintering and high temperature mechanical strength of corundum based castables were investigated. The results show that the presence of reactive alumina with high specific surface area leads to accelerating of the hydration of calcium aluminate cement, thus shortening the working time and setting time of the castables, which can be ascribed that the critical energy barrier for stable nuclei of hydration products of cement can be reduced by the high specific surface area of reactive alumina. The sintering of the corundum based castables can be accelerated by the reactive alumina with high specific surface area and high amount of impurities, however, the reactive alumina with too high specific surface area and impurities can also lead to noticeable shrinkage of castables. In addition, high temperature mechanical strength of corundum based castables can be decreased by the higher amount of trace impurities of reactive alumina due to formation of low-melting phase at high temperatures.