The kinetic modeling of oil cracking by hydrothermal pyrolysis experiments using a Japanese oil, derived from type II kerogen, is reported here. Pyrolysis experiments were carried out in the temperature range from 370 to 410 degrees C and the time range from zero to 384 h. The chemical structural analysis of the product oils by H-1 NMR and by elemental analysis provided important information for the kinetic modeling. The kinetic model to characterize changes in amount of hydrocarbon gas, light saturates, light aromatics, heavy saturates, heavy condensed aromatics, heavy non-condensed aromatics and coke was finally constructed. The apparent activation energy for cracking of heavy saturates was found to be 76 kcal/mol, which is close to a value published on hexadecane cracking under high pressure in anhydrous conditions. Application of the model to geological conditions shows that the cracking of heavy saturates occurs within a temperature range of 190 to 230 degrees C, which is relatively a higher range than that usually accepted in petroleum geochemistry.