Biomass as feedstock for renewable energy and biomaterials production is of great importance to tackle energy, economic and environmental issues. Biomass can be processed in several ways depending on its composition, moisture content and availability. Hydrothermal Carbonization (HTC) is one possible option to deal with the biomass streams. In this study, oat was processed in a lab-scale stirred-batch HTC reactor to evaluate the effect of reaction temperature and residence time on the composition and yield of hydrochar obtained during the process. The results demonstrate that these operating parameters strongly affect the characteristics and the amount of the hydrochar produced. The results indicate that the increasing of the HTC severity conditions produces an enrichment of hydrochar in carbon content up to 72.8%. On the other hand, the hydrochar yield decreases from 0.85 to 0.56 g/g as the severity factor increases from 0.11 to 0.37.