In the present article, some studies on the pulse radiolysis of high temperature and supercritical water are summarized. The temperature and density effects on the absorption spectrum of hydrated electron, the yield of water decomposition products, and reaction rate constants are addressed and discussed. Meanwhile, the temperature effects on the absorption spectra of solvated electron at elevated temperatures have been also stated. These investigations are expected to be useful for the modeling of water radiolysis in supercritical water cooled reactors (SCWR) and for a better understanding of fundamental knowledge in radiation chemistry as well.