Abstract: The isothermal gasification tests of 4 biomass samples were performed in CO₂ atmosphere using a thermo-gravimetric analyzer at 750~1 000 ℃. The results show that the gasification reaction rate increases with increasing gasification temperature. Through the dimensionless procedure of r using r_0.2, two different trends of dimensionless gasification reaction rate with carbon conversion are observed when the gasification temperature is changed. Accordingly, activation energies of the biomass chars were calculated by iso-conversion method. It is found that the activation energy of any biomass char is basically constant, not varying with the carbon conversion obviously. Activation energies of different chars are mainly affected by the inorganic mineral composition in the char and can be described in a linear function of the ratio of W_C/W_FC, which is E=233.9-1 005.7×(W_C/W_FC). It can be predicted that the intrinsic activation energy of the biomass char might tend to a certain value 234 kJ/mol if the catalytic effect of the metallic element is ignored.