Sweet potato starch was modified using Thermus aquaticus $alpha$-1,4-glucanotransferase ($Ta{alpha}GT$), and its structural and rheological properties were investigated. $Ta{alpha}GT$-modified starch had a lower amylose level and molecular weight than raw starch. The chain length distribution showed an increased number of short and long branched chains and the formation of cycloamyloses. Compared with raw starch, $Ta{alpha}GT$-modified starch displayed a lower gelatinization enthalpy and a wider melting temperature range. The X-ray diffraction of $Ta{alpha}GT$-modified starch was a weak V-type pattern with distinct sharp peaks at 13 and $20^{circ}$. Scanning electron micrographs of modified starch exhibited big holes on the surface and the loss of granular structure. The frequency sweep measurement revealed that the gel of $Ta{alpha}GT$-modified starch was more rigid than raw starch gel. However, the structure of modified starch gel was destroyed by heating at $75^{circ}C$, and a firm gel was re-formed by subsequent storage at $5^{circ}C$, indicating thermoreversible property.