The liquefaction mechanism of solid cellulose particles in hot compressed water is estimated experimentally and theoretically. Experiments are carried out using a batch reactor that equips a temperature controller. The heating profile is controlled to make a proportional relationship with time. Cellulose is drastically decomposed and liquefied when the maximum temperature is set over similar to 513 K. It decomposed to oligosaccharides, monosaccharides, and pyrolysis products such as glyceraldehydes and 5-hydroxymethyl-2-furaldehyde. The concentration of oligosaccharides and monosaccharides shows maximal values for the maximum temperature set. The theoretical treatment of cellulose decomposition is investigated for stepwise processes of decomposition from ( 1) cellulose to oligosaccharides, ( 2) oligosaccharides to monosaccharides and pyrolysis products, and ( 3) monosaccharides to pyrolysis products. The theoretical equation derived for the proposed mechanism inserts the temperature variation with Arrhenius relationship. The concentration profiles on maximum temperature are calculated numerically. The calculated results show a reasonable correlation to the experimental data.