Abstract: To determine the kinetic characteristics of piled combustion of biomass or bulk char,the oxidation experiments of different biochar including maize stover char,rice husk char,elm bark char and cypress char were conducted in a simultaneous thermal analyzer.In these experiments,crucibles with different cross sectional area（0.21,0.42 cm~2）and samples with different height（full crucible,1/10crucible）were used to analyze the mass loss rate. The results show that the mass loss rate is proportional to reaction area and irrelevant to sample size,indicating that the piled combustion of biochar is surface reaction. Thermogravimetric data of cypress char can better reflect kinetic characteristics due to its lowest ash content and diffusion resistance of oxygen,and its oxidation kinetics based on surface reaction are obtained as the frequency factor AS= 40.5 m/s and activation energy E = 61.5 kJ/mol.