We propose a new type of environmentally friendly system called the "Organic Rankine Cycle" (ORC) in which low-grade heat sources are utilized. This system combines a circulated thermosyphon with a turbine system. The working fluid used in this study is an organic substance which has a low boiling point and a low latent heat for using low-grade heat sources. A numerical simulation model of the ORC is made in order to estimate its optimum operating conditions. An experimental apparatus is also made in this study. From the numerical simulation, it is suggested that HCFC-123 gives higher turbine power than water which is a conventional working fluid, and operating conditions where saturated vapor at the turbine inlet would give the best performance. From the experimental results, HCFC-123 improves the cycle performance drastically. In addition, the turbine made for trial use in this study gives good performance.