The oat grain is often subjected to different hydrothermal treatments to inactivate hydrolytic enzymes that can have undesirable effects on the end-product quality. Hydrothermal treatments may affect the functional properties of oat starch. The objective of this work was to evaluate the effect of eight different hydrothermal treatments on the physicochemical and digestibility properties of oat starch. Hydrothermal treatments were selected based on heat treatment applications in oat industry and listed as HT-1) Ethanol boil HT-2) Ethanol boil and toasted HT-3) Steamed at 106. °C HT-4) Steamed at 106°C and toasted HT-5) Covered autoclaved at 120. °C HT-6) Uncovered autoclaved at 120. °C HT-7) Covered autoclaved at 130. °C and HT-8) Uncovered autoclaved at 130. °C. The morphology of the oat starch was altered as a result of various heat treatments. HT-5 and HT-7 treatments had more large starch granules than the other samples, however the apparent molecular weight of the starch from these samples was unaffected by the treatment. HT-6 and HT-8 resulted in the most significant changes to the gelatinization properties of the oat starch. HT-6 oat sample had significantly (P< 0.05) lower paste viscosity than starches from the oat samples with other treatments. The in vitro starch digestibility increased significantly (P< 0.05) for the HT-6 and HT-8 oat samples. This study demonstrated that thermal treatments can cause reorganization of the amylose and amylopectin chains that can result in changes in their physicochemical and digestibility properties. © 2013.