In this study, Erianthus arundinaceus energy grass was examined by conducting hydrothermal carbonization (HTC) using a laboratory-scale autoclave in the temperature range of (180-240 degrees C) and the retention time of (0-120 min). The parent material and hydrochars were investigated in terms of mass yield, proximate and elemental analyses, the higher heating value (HHV), X-ray diffraction, Fourier transform infrared, and X-ray fluorescence (XRF) analyses. Furthermore, CO2 gasification was conducted on the pyrolysis char prepared from the parent material and the hydrochars using thermogravimetric analysis. It was found that increasing the HTC temperature and residence time caused the mass yield to decrease and the HHV to increase. Hydrochar crystallinity increased with HTC temperature up to 210 degrees C as a result of the deformation of the amorphous components, while under 240 degrees C, the crystallinity declined from its peak at 210 degrees C due to the commencing of cellulose degradation. The XRF analysis showed that the alkali index was increased due to increasing temperature and residence time. Gasification reactivity of the pyrolyzed hydrochar prepared at 180 degrees C was enhanced due to longer residence time (120 min), while those from very short residence time (0 min) showed a weaker reactivity than the parent material. Changing the HTC temperature showed minimal effect on the reactivity. At higher HTC temperature (240 C), the catalytic activity was restricted by the highly ordered structure. This was not the case at 180 degrees C and long residence time of 120 min, where the reactivity was mainly controlled by the increased minerals.