The effect of volume ratio between acetic acid and acetic anhydride on the degree of substitution(DS) was investigated for the synthesis of starch acetates by microwave assistance.Chemical structure、granule morphology、crystalline、thermal properties and molecular weight distribution of starch acetates were all determined separately by infrared spectrum(IR)、scanning electronic microscope(SEM)、X-Ray Diffractomer(XRD)、differential scanning calorimeter(DSC)and gel permeation chromatography(GPC).The physicochemical characteristics for starch acetates synthesized under different acetic acid to acetic anhydride ratio were also compared on the basis of those results.The results were obtained as follows:the ratio between acetic acid and acetic anhydride had a great effect on DS;DS was reached to the highest of 2.92 at the ratio of 1∶ 1(volumn ratio);and then,DS decreased along with the increase of acetic anhydride.Moreover,the chemical structures and physical-chemical properties had a great changes after the esterification.The hydroxyl groups of each glucose unit were replaced by acetyl groups gradually as the increase of DS.As it is shown from SEM results,the surface of starch acetate became rough and porous.Meanwhile some of the starch particles have been cracked into tiny parts;After modification,new crystal formed and old intrinsic crystalline structure destroyed.As a result of that,the degree of crystallization decreased.The thermal properties of starch acetates have also changed greatly with the decline of glass transition temperature(Tg) and melting temperature(Tm).Therefore,starch acetates are expected to exhibit better thermoplasticity than native starch.The molecular weight of starch acetates were decreased and its distribution became wider,this indicated that starch chain was broken during esterification.