We have realized chemical recovery of a phenolic compound, 2-(hydroxy-benzyl)-4-methyl-phenol (BMP), from organosolv lignin without forming char in a mixture of supercritical water and p-cresol (Okuda, K.; et al. J. Phys.: Condens. Matter2004, 16, S1325). In this paper, we evaluated the reaction rate constants of the depolymerization process of organosolv lignin using Monte Carlo simulation. We also evaluated the formation and decomposition rate of BMP based on the proposed reaction scheme. The obtained reaction rate constants gave insight into the mechanism why the mixture of supercritical water and p-cresol suppressed the formation of char. They also suggested the reaction path for the formation of BMP. In addition, these reaction rate constants enable the design and optimization of the chemical recovery processes to realize the highest yield.