Cryptomeria japonica (CJ) samples were heated in a thermogravimetric analyzer (TGA) with CO2 as agent to study the gasification kinetics in the isothermal process. The samples were torrefied at 250 and 350 °C for 1 h. before being tested in TGA. Three isothermal conditions of 750, 800, and 850 °C were tested in this study to estimate the kinetic parameters of the Arrhenius form with the proposed models. The homogeneous model (HM), shrinking core model (SCM), and linear model (LM) were used, and the predicted results obtained from these models were compared with experimental data. The reaction rate of gasification was enhanced as temperature was raised, and a correlation of kinetic parameters with temperature was obtained. The simulated results of the linear model (LM) fit well with experimental data. This work is useful for design gasification in downdraft gasifier.