Abstract: In order to analyze biomass pyrolysis in down flow reactor, computation of flow and heat transfer for solid media and biomass mixture in the inclined tube was investigated theoretically. A two stage model (sliding stage and fully developed stage) was suggested and equations on flow and heat transfer in these stages were written. The model was applied to study the influence of structural and operational parameters of the reactor. Result shows that under typical condition, heat transfer completed immediately after the mixture entering the tube; Heat media size and inclined angle of the tube have significant effect on terminal temperature difference between heat media and biomass powder.