Abstract: With establishment of a unified hydraulic model for both natural and forced circulation loops,and with considering the unique variation of thermo-physical properties at supercritical pressures,along with the nonlinear momentum transport and system coupling characteristics,this paper,by applying appropriate nonlinear numerical computation based on continuation concept,presents a systematic comparative study on the complex hydraulic features of both supercritical water systems.Special non-monotonic relationships of mass-flow rate and heating power of both natural and forced circulation,with their mechanisms and features,are revealed and comparatively investigated.Related thermal energy transmission issues of both systems are analyzed as well.Meanwhile,effects of such factors as inlet bulk temperature of heating section,local resis-tances and/or pump characteristics on system hydraulic and heat transmission features are delivered.Further-more,characteristics of forced circulation driven by both a real and ideal pump are compared.