The changes of structure and chemical reactivity of ultrasound-treated cornstarch (UTCS) granules are presented and discussed. It was found by polarized light and scanning electron microscopy (SEM) that the crystalline structure of UTCS granules did not change, but the amorphous area was slightly destroyed. Pores or channels were detected in the UTCS granules. X-ray diffraction revealed that UTCS had the A-type crystalline pattern of native cornstarch, which were not markedly changed. The gelatinization temperature and enthalpy of gelatinization (ΔH) of UTCS first increased and then slightly dropped with increasing degree of hydrolysis (DH). An increase in the DH of UTCS from 0 to 3.1 % resulted in an insignificant change in the degree of substitution (DS) and reaction efficiency (RE). Further increases in the degree of hydrolysis up to 15.7 % resulted in an obvious increase in the DS and RE, but increase in the DH to even higher values drastically decreased DS and RE. Compared with hydroxypropylated cornstarch, the pasting temperature of hydroxypropylated UTCS increased, but UTCS exhibited a lower maximum viscosity. The breakdown of hydroxypropylated starch paste was reduced after ultrasonic treatment before hydroxypropylation. The setback value increased after ultrasonic pretreatment when the degree of hydrolysis was < 7.8 % but decreased at DH > 7.8 %.