Natural convective flow of supercritical fluids has become a hot topic in engineering applications. Natural circulation thermosyphon (or NCL: natural circulation loop) using supercritical/transcritical CO2 can be a potential choice for effectively transportation of heat and mass without pumping devices. This chapter presents a series of numerical/experimental investigations into the fundamental features in a supercritical/transcritical CO2 based natural circulation loop systems as well as possible applications and innovations in engineering fields. New heat transport model aiming at transcritical thermosyphon heat transfer and stability is proposed with supercritical/transcritical turbulence model incorporated. The effects from various system parameters, operation conditions, accident analysis, apparatus developments as well as control strategies are also included with detailed explanations in this chapter. It is clearly found that such novel fluids and systems would be one promising candidate for future development of energy solutions to global warming issues.