The hydro-thermal cracking of heavy oils, such as Canadian oil sand bitumen and Arabian heavy vacuum residue, as well as their model compound, was performed over sulfided Ni/Al2O3 and NiMo/Al2O3 catalysts under 663-703 K and 5.0-8.0 MPa of hydrogen pressure in a batch autoclave reactor. According to the reaction mechanism of hydro-thermal cracking, some free radical initiators, such as di-tert-butyl-peroxide (DTBP), sulfur, etc., were added into the feed to generate free radicals at lower temperature, and some initiators did obviously show a promotional effect on the conversion of hydrocarbons. The reaction mechanisms of hydro-thermal cracking as well as the enhancing effect of initiators were studied by a probe reaction with 1-phenyldodecane as a model compound under the conditions of hydro-thermal cracking. The hydro-thermal cracking of hydrocarbons proceeded via a free-radical mechanism and hydrogenating quench. The initiators might easily generate free radicals under the reaction temperature, these radicals might abstract H from hydrocarbon molecules and reasonably initiated the chain reactions, therefore, promoted the conversion of hydrocarbons even at lower reaction temperature. The reaction temperature could be lowered by the addition of a free radical initiator, while keeping the same conversion level.