In order to handle high-level tritiated water (HTO) safely, the self-decomposition behavior has been investigated as functions of tritium concentration (from 16 GBq/cm3 to 2 TBq/cm3) and storage temperature (269K ~ 303K). The representative decomposition products such as H2 in the gas phase and H2O2 in the liquid phase were measured periodically, storing HTO in a leak-tight vessel. The effective production rate of H2 increased with tritium concentration, however, the normalized production rate by tritium decay, like effective G-value, decreased with tritium concentration. The effective production rate of H2O2 also increased with tritium concentration and the normalized one also decreased under consideration of its natural decomposition rate, though it thought that the almost H2O2 calculated by the reported G-value decomposed by extra stimulus in tritiated water. The effective production rates of H2 and H2O2 increased with temperature.