Abstract: Well-crystallised flat hexagonal nano alumina was prepared by hydrothermal synthesis method with addition of sodium nitrate additive. Changing the quantity of sodium nitrate to observe the transformation of alumina shape and its characters was conducted. The samples are characterized by BET, XRD, TEM, HRTEM and FT-IR techniques. The experimental results indicate that crystalline structure of alumina and boehmite are not changed by sodium nitrate addition and their quantities, but the profiles of alumina were larger than that without sodium nitrate additive, and the area of crystal planes (100) and (111) are increased. The larger quantity of sodium nitrate additive will make the area and pore volume of alumina decrease and the pore size increase.