Abstract: A thermogravimetric method under constant temperature replacing the beginning inert gas with burning air was established in this study for determining the combustion kinetics of single char particle. The combustion process of three types of coals, Fujian Longyan coal, Fujian Yong’an coal and Shandong coal, with diameter of 3.0—4.5 mm, in thermobalance under 900℃ was discussed using the familiar shrinking-core model.While the combustion of single char approached the end, the outside and the ash layer diffusion resistances tend to be zero as the reaction resistance dominated in the total resistance of combustion.Therefore, at this stage the inherent oxidation reaction constant (k_R) of the above char samples were estimated by analyzing the total resistance of combustion.The good agreement of combustion reaction rate between data determined in this study and calculated values obtained from reference in the drop tube furnace system, implied that the inherent oxidation reaction constant (k_R) determined by the thermogravimetric method under constant temperature was satisfactory and valuable. The gas diffusion through the ash layer of char particle during combustion process was discussed also. At the ash layer thickness half the char radius, the ash layer resistance in the combustion process reached a maximum value, at which the total resistance of combustion,1/K,was less than 25 s•m^-1 and would be close to the reaction resistance,1/k_C，approaching the end of combustion. So the effects of gas diffusion resistance through ash layer on char combustion process could be ignored during the whole combustion process.