Gasification is one option for producing cleaner fuel from biomass. A gaseous mixture of H2, CO, CH4, and CO2 is produced through the partial oxidation of biomass with a gasifying agent such as air, pure O2, steam, CO2, or a mixture of these. This method is capable of handling a wide range of inhomogeneous biomass (including forest, agricultural, and organic processing residues) and converting them into a homogeneous gas with a considerably higher level of applicability. In this research, the CO2 gasification of hydrothermally treated biomass has been studied using TG-DTA analyzer (Bruker TG DTA 2000SA) apparatus. The biomass treated was a mixture of corn cob and coconut shell (weight ratio of 1:1). This raw biomass was firstly subjected to hydrothermal treatment at three different temperatures (200, 240, and 270°Cdenoted as H-200, H-240, and H-270) using a batch autoclave prior to being gasified by CO2 under atmospheric pressure in the TGA apparatus. The experimental results show that the weight loss of hydrochar was resulted mostly from the process of devolatilization (82.9286.16%). Hydrochar obtained from higher hydrothermal temperatures demonstrated a lower reactivity of gasification, due to the lower amount of moisture and volatile matter. In addition, higher-temperature hydrochar contained lower potassium content and thus shifted the conversion of gasification reaction to a higher temperature.