Fast pyrolysis of biomass materials is an effective means to convert biomass into useful energy products. The conversion process can be significantly affected by the properties of the biomass particle and the operating conditions. To obtain a better understanding of this process, a direct numerical simulation method was proposed and used to conduct particle-scale simulations. In this study, the lattice Boltzmann method was employed to solve the flow field and the intraparticle transport of heat and mass. A multistep pyrolysis kinetics mechanism was used to describe the chemical reactions that convert solid biomass tcr gaseous and solid products. The predicted evolutions of center temperature and solid mass fraction agreed 'well with the experimental data. The validation demonstrated that the present model was capable of revealing the detailed conversion process of biomass fast pyrolysis at the particle scale. Parametric studies were conducted to characterize the effects of particle size, particle aspect ratio, inlet gas temperature, and reactor wall temperature on the conversion time and final product yields. The numerical results showed that the conversion time increased as the particle size increased and decreased as the inlet gas temperature and reactor wall temperature increased. When the particle size was decreased, more tar and syngas were produced while less char was generated. The same trend offinalproduct yields was also found when the inlet gas temperature"and. reactor wall temperature were increased. The results also indicated that the temperature gradients inside the particle can be neglected under certain particle size, i.e., equal to or less than '0.2 mm under the conditions studied. The heat flux from the reactor wall was found, to be more significant to the fast pyrolysis process than the inlet gas temperature.