In this study, hemp stem was used to prepare high surface area activated carbon (AC) by KOH activation. The structure, characterization, thermal and structure analysis on reaction mechanisms of AC were investigated via N2 adsorptionâdesorption isotherm, Fourier transform infrared (FTIR) spectroscopy and thermogravimetric-mass (TG-MS) spectrometry. Results show that when the impregnation ratio is 4.5:1 (KOH/char), the activation temperature is 800 °C, and the activation time is 1.5 h, AC has the highest specific surface area of 2388 m2·gâ1 and exhibits narrow pore size distributions with maxima in the micropore areas. The reaction mechanism of AC from hemp stems by KOH activation is as follows: In the first carbonization stage, it is primarily because of the substitution, scission, and oxidization reactions of methylene. Then in the second activation stage, it is mainly related to polycyclic reactions, reactions between carbon and KOH, and reactions between intermediate potassium oxide species and carbon. Keywords: Hemp stem, Activated carbon, Characterization, Reaction mechanis