Jet fragmentation greatly influences the possibility of steam explosion and the formation of a debris bed when a molten corium jet falls into subcooled coolant during a severe accident of a nuclear reactor—which is called fuel and coolant interaction (FCI). The characteristics of different jet fragmentation mechanisms and the conditions under which they play a major role are still in doubt. Experiments were carried out to investigate the fragmentation characteristics of melt jet interaction with water at medium temperature (~680°C) and high temperature (1800°C to 2150°C). Molten metal [tin or Type 304 stainless steel (304SS)], oxide (alumina), and their mixture (304SS-alumina) were used as melt materials to obtain different fragmentation mechanisms. In addition, the effects of melt temperature, water subcooling, and water depth on jet fragmentation were also studied. Through comprehensive analysis of high-speed photography, dynamic pressure, water temperature variation, and jet breakup length during interactions as well as the morphology and size of debris after interactions, it was found that the characteristics of jet fragmentation varied greatly at different melt temperatures and water subcooling due to competition between hydrodynamic fragmentation and thermodynamic fragmentation caused by boiling. In addition, under high-temperature conditions, fragmentation of alumina was much greater than 304SS due to the fracture of solidifying melt caused by thermal stress. Finally, five kinds of mechanisms of melt jet fragmentation under different conditions are summarized.