Under supercritical conditions, polyethylene(PE) decomposition experiments were conducted in a batch reactor, and the reaction mechanism and effects ol supercritical water were examined. According to analysis of experimental data, the mechanism of PE decomposition is based on random break of carbon-carbon(C-C) bonding. During C-C breaking, PE molecules are decomposed into small oils molecules which contain paraffin and olefins, and then aromatic compounds are produced by polymerization of olefins. NMR measurement showed that hydrogen of H2O is incorporated into the products under supercritical condition of water. A fundamental model of PE decomposition is proposed, and its performance is discussed by comparing calculations and experimental data. Additionally. reaction rates of coking are evaluated as quite small compared with PE thermal degradation.