Abstract: In this paper,activated alumina is prepared by precipitation method where sodium carbonate and aluminum chloride are used as raw materials and high temperature phase transformation of lanthanum oxide doped alumina was studied.Thermal behavior of the sample was analysed by differential thermal analysis,phase transformation and surface morphology were analysed by XRD and TEM.The results show that doped lanthanum can inhibit the phase transformation and reunite of alumina during the heating process,maintain a high specific surface area of sample.