The acetylated starches from corn were prepared under ultra high pressure (UHP) by acetic anhydride as reaction reagent, and NaCl as the reaction medium. The structure and properties of acetylated starches were analyzed by using light microscope, X-ray diffractometer and rapid viscosity analyzer. The results indicated that the granular acetylated starch showed a similar crystallite structure as that of native starch. In addition, the acetylated starches were gelatinized when treated at 600 MPa and the structure of starch granules was gradually destroyed, whereas the crystal structure was transformed from the A-type crystallite to the V-type crystallite. However the gelatinization negatively affected the degree of substitution of acetylated starch. The appropriate amount of NaCl favored the maintenance of starch granular structure under UHP, which would effectively increase the degree of substitution of acetylated starch. The degree of substitution and peak viscosity of acetylated starch reached the highest value (0.090 and 400.00 cP, respectively) when corn starch was treated at 400 MPa with the concentration of the solution at 1.0% and the amount of acetic anhydride at 2.0%.