The Al/AlN bonded corundum based multiphase material was prepared by raw materials of fused alumina and metallic aluminum powder by in situ reaction at 1100°C for 3h in N2 . The XRD analysis showed that the mineral phases of prepared material are corundum, metallic aluminum and AlN respectively. The SEM investigation revealed that both tetragonal whiskers and hexagon powders of AlN were formed. The results of hydration tests indicated that the rate of weight gain and pulverization of the material were negligible and the ratio of residual crushing strength was kept at a high level. The XRD patterns of samples after hydration tests identified that the AlN phase still remained although a small amount of AlOOH and Al(OH)3 was formed due to the hydration of AlN. It is believed that the limited hydration of AlN should be contributed to the dissolution of O element to AlN. The hydration mechanism of AlN was discussed.