Hydrolysis experiment of woody biomass in hot compressed water (HCW) was conducted in a batch-type reactor and the characteristics of solid residue after the HCW hydrolysis treatment were measured to understand the hydrolysis mechanism of woody biomass. Woody slurry of Douglas fir was used in this HCW experiment. Reaction temperature was controlled from 423 to 523 K, and the reaction time was set at 1 min. After separation of solid residues from the hydrolysis liquid material, the solid residue characteristics, such as proximate and ultimate analysis, particle size distribution, particle shape, surface area, pore size distribution, chemical composition, and equilibrium moisture content ratio, were measured and then the decomposition mechanism of woody biomass in HCW was discussed. It was found that the solid residue characteristics yielded in HCW changed drastically depending on the reaction temperature. For example, cellulose crystallinity decreased with increasing reaction temperature, and the physical characteristics of solid residue, such as particle shape, particle diameter, and pore size distribution, also changed dramatically depending on the reaction temperature. The physical or chemical characteristic change in HCW was strongly related to the decomposition of components of woody biomass at that reaction temperature. Dehydration of woody components was one of the most important factors to understand physical characteristics during HCW treatment.