This paper has two parts: The first part covers the review of previous researches on biomass fractionation based on mechanistic and kinetic viewpoints. Since the major components of biomass are cellulose and lignin, fractionation of each component is discussed. Kinetics and reaction mechanism of cellulose hydrolysis, glucose fractionation, and decomposition are also summarized. Based on the results of these basic studies, new processes of aldehyde recovery from cellulose/glucose and cresol addition method for fractionation from lignin are introduced. The second part of this paper proposes a new approach (original research) to decompose lignin and biomass with CeO2 nanocatalysts fabricated by supercritical hydrothermal synthesis method. Char formation decreased, and liquid product yield increased when the nanocatalyst, {001} surface exposed cubic CeO2 was used. This is probably because of the suppression of Friedel-Crafts reaction due to the oxidation of aldehydes, which could be the bridge molecules of phenolic structures.