Cellulose is the most abundant nature polymer and the main component in all biomass. In this study, the degradation kinetic parameters and structure changes of microcrystalline cellulose (MCC), which served as fundamental studies for degradation of biomass in subcritical water, were investigated at the temperature range from 100 to 300 degrees C. The yield of the MCC residue began to decrease at 205 degrees C and reached the lowest value at 275 degrees C. However, it howed an increase at the temperature higher than 275 degrees C. The degradation area of MCC (205275 degrees C) was separated into zones 1 and 2 with 245 degrees C as a boundary. The activation energy (E), pre-exponential factor (A), and reaction order (n) of MCC in each zone were 226.5 kJ mol(-1), 2.3 x 10(23) s(-1), and 0.6 (zone 1) and 423.1 kJ mol(-1), 9.0 x 10(40) s(-1), and 0.5 (zone 2), respectively. There showed a breaking point of 245 degrees C for the Arrhenius plot in the reaction area. The surface morphology of the MCC residue had no significant change below 260 degrees C, as indicated by scanning electron microscopy (SEM) images. However, it was completely destroyed at the temperature above 275 degrees C, and MCC residues with strong pore structures were obtained at higher temperatures. The structures of the crystalline region of the MCC residue below 275 degrees C had no significant change, although 89.2% MCC was degraded at 275 degrees C. However, they nearly disappeared at higher temperatures with the steeply reducing crystallinity index of MCC at the same time. As for the degradation mechanism of MCC, it was proposed that only the surface of MCC was degraded at 205-245 degrees C and the hydrogen and glycosidic bonds on the interior of MCC fibrils were destroyed at the temperature range from 245 to 275 degrees C. Finally, the remaining MCC as well as generated oligomer and monomer were further degraded at higher temperatures.