The physicochemical properties of the starch from Taiwan’s water caltrop (Trapa taiwanensis Nakai) during growth were investigated. The results showed that the starch content of water caltrop increased from 39.7% to 74.7% on a dry weight basis as growth progressed (14th to 42nd day). The shape of the starch granules was oval and polyangular during growth period. The particle size of starch increased with the increase of from 15.31 to 24.16 μm with physiological aging. The X-ray diffraction patterns could be classified as a typical A-type crystalline structure. The gelatinization onset temperature (To), peak temperature (Tp), and gelatinization enthalpy (ΔH) increased with increasing growth time. The results of rapid viscosity analysis of water caltrop starch showed that the pasting temperature, peak viscosity, final viscosity and setback value increased during growth. The increase in starch granular size could be the major factor influencing starch physicochemical properties during maturity.