Heat and mass transfer have been investigated during fast pyrolysis of biomass powder packed bed. For cellulose powder, temperature at the center of the bed, T_c, increased monotonously with time due to thermal conduction for T_c<500K. The temperature increased slowly with time due to heat of endothermic cellulose decomposition for 500 K<T_c<600 K. On the other hand, the exothermic behavior was observed at T_c>600 K. For lignin powder, only exothermic behavior was observed at T_c>600K. Therefore, it was found that the exothermic behavior could be occurred by the tar cracking. Furthermore, the relationship between heat flow rate and temperature was studied in order to investigate the heat of reactions. The endothermic and exothermic behaviors could occur for woody biomass because the heat flow rate had the minimum value at T_c=600 K and the maximum value at T_c=650K for both powders.