An isothermal operation is implemented by employing a thermogravimetric analyzer (TGA) for simulating the thermal decomposition behavior of 58 mu m pine sawdust in air atmosphere. An independent parallel reaction model is adopted in this study to describe the thermal decomposition mechanism. The Weibull distribution function is used to record and analyze the weight loss during isothermal decomposition at different temperatures (500, 600, 700, and 800 degrees C). The total weight loss of the pine sawdust is assumed as a linear combination of individual weight loss from three components, including the char and two volatile matters. The plot of the thermal decomposition rate curve leads to kinetic parameters such as the reaction rate constants and the reaction order. The results show that the Weibull distribution function successfully represents decomposition curves of three components, and fits the experimental data very well. Therefore, this study provides a simple way to evaluate the decomposition rate of biomass combustion in a real combustor.