The optimal condition for the torrefaction of mixed softwood was investigated with respect to the reaction temperature and time. While the reaction time had a strong impact on the energy yield of torrefied biomass, the effect of temperature was considerably lesser, under the torrefaction conditions used in this study. At a low SF, torrefaction had a significant effect on the degradation of hemicelluloses, but not on cellulose or lignin. Therefore, when the weight loss of biomass by torrefaction is considered, introducing low SF values for mixed softwood is appropriate for the torrefaction of biomass for the in production of high energy density fuels.