In the new century, global warming and energy problems are becoming more critical. The practical use of Ocean Thermal Energy Conversion (OTEC) is refocused because it is possible to supply stable electric power and a variety of integrated applications. It is clear that the system performance of the double-stage Rankine cycle (D-R) is higher than that of the single-stage Rankine cycle (S-R) from a theoretical study. However, the characteristics of the D-R have not been sufficiently clarified in experimental studies. So, it is necessary to perform experiments involving D-R. This study examined the decrease in irreversible loss in the heat exchange process of D-R experimentally compare to S-R. The entropy generation rate of D-R is lower than S-R. The power output of D-R is expected to be higher than that of S-R.