The aim of this study is to observe conversion of cellulose, which is the main compound of biomass, into its building-block chemicals in hot-pressured water as reaction medium with no addition of organic solvents. Water in liquid state under temperature and pressure above boiling point and below the critical point (374. 15 ºC and 22.1 MPa) is called as hot-pressurized water (or sub-critical water). Nowadays the biomass has great attention across the World as renewable source of energy, at the background of the quickly growing energy demand, since it is widely available and cheap. This technology is totally environmentally friendly and uses water as a reaction medium. As well known, since plant biomass contains up to 50% cellulose, it was decided to use it as a model compound in this study. Decomposition of cellulose leads to formation of various compounds. Among them levulinic acid is the most attractive chemical. Moreover, this acid marked as “Top 12 Building Blocks” of most perspective chemicals and obtaining from biomass by hydrothermal treatment is not widely studied. During this study, different reaction parameters such as temperature, pressure, reaction time and external oxidizer addition were studied to clarify their effects on cellulose decomposition and product yields to achieve the highest selectivity of the desired product. Addition of H2SO4 led to increase cellulose conversion up to 73% at 200 oC with a H2SO4 concentration of 125 mM at 60 min reaction time. Under same conditions, the yield of levulinic acid was successfully achieved to 38% after 60 min.