The two-step mechanism reaction model is widely used to represent solid degradation during the torrefaction process at fixed temperature. However, solid degradation during the heating period was neglected and the lack of the model for predicting volatile composition. Thus, the original model was extended to cover heating and isothermal stages during the torrefaction process based on thermogravimetric analysis using empty fruit bunch as case study. Torrefaction experiments were conducted at different temperature ranges for various heating rates. Based on the results, a good agreement between experimental and extended model was obtained with an absolute error of < 1 % indicating the high reliability of model prediction in estimating volatile composition.