Low-grade Thermal Energy Conversion (LTEC) is expected as a solution for global-scale energy and environmental problems. Using a multi-stage Rankine cycle is gathering attention as one of the methods to improve the system performance. On the other hand, even if total heat transfer areas of multi-stage and single heat exchangers are equal, pressure loss of multi-stage heat exchangers is larger than that of single heat exchanger. In this study, the pressure loss of multi-stage heat exchangers from the viewpoint of the net power of the power generation system, including the pump power for the heat source, is considered to develop an assessment method for LTEC using multi-stage Rankine cycle.