Abstract: Biomass is regarded as a promising low-cost precursor for the preparation of activated carbons. However, direct carbonization of biomass usually produces a low-surface-area or even non-porous carbons that are useless for CO₂ capture. In this work, garlic peel was first transformed to a hydrochar by hydrothermal carbonization and then chemically activated by KOH to obtain activated carbons with high-surface-areas and large pore volumes. The microstructure and morphology of the activated carbons were characterized by N₂ adsorption, SEM and XRD. Results indicate that their surface area and pore volume are mainly determined by the activation temperature and KOH/hydrochar mass ratio. Activated carbon (AC-28) obtained by KOH activation with a KOH/hydrochar ratio of 2 at 800℃ has a well-developed porosity with a surface area and pore volume of 1262 m²/g and 0.70 cm³/g, respectively, while a reduction of the activation temperature to 600℃ (AC-26) results in a material whose corresponding values are 947 m²/g and 0.51 cm³/g. Although AC-26 exhibits a much lower surface area and pore volume compared with AC-28, it has the larger CO₂ uptake of up to 4.22 mmol/g at 25℃ and 1 bar due to its higher microporosity of up to 98% and abundant narrow micropores, implying that the microporosity is one of the main factors for CO₂ capture besides the traditionally-believed surface area and pore volume. The isosteric heat of CO₂ adsorption indicates that the affinity between the activated carbon and CO₂ molecules increases with the volume of narrow micropores less than 0.8 nm and the number of surface oxygen-containing functional groups.