In the present study, the main results of thermodynamic analysis of an actual cycle of an ethylene reliquefaction plant are provided. The research direction is to analyze the efficiency of the on-board ethylene reliquefaction plant cycle in order to comply with the mandatory ship energy efficiency standards (EEDI, SEEMP). Experimental data from the recording of cargo operations monitoring during the operation of the reliquefaction unit on the gas carrier LEG “ANTIKITIRA” were used for the analysis. Boil-off gas (BOG) is reliquefied in a cascade refrigeration plant. The overall installation cycle consists of two interconnected two-stage cycles. The working fluid of the bottoming cycle is the transported cargo (ethylene). The working fluid of the topping cycle is propylene. The research is based on the determination of irreversible losses in the reliquefaction plant cycles by the entropy-cycle method of thermodynamic analysis. A feature of the method used is the accounting for changes in the flow rates of working fluids both in the bottoming and topping cycles, as well as in compression processes from stage to stage. It is proved in the study that for a group of sequentially combined cycles the joint effect is equivalent to the effect of a cycle between extreme temperatures. Based on the results of the analysis obtained by the entropy-cycle method, the impact of the process excellence in the main components on the reliquefaction plant energy efficiency was evaluated. The greatest thermodynamic irreversibility from the total energy losses is observed in the processes of compression in the two-stage compressor of the bottoming cycle (9%), total throttling in the reliquefaction plant (8.5%), and vapor superheating at the suction into the low stage of the two-stage compressor of the bottoming cycle (8%). In addition to the analytical determination of irreversibility the method provided a clear analysis. The results of the study allow designers and operators of gas carriers to determine the components of the reliquefaction plant that require improvement and take practical measures to improve the operational characteristics of the reliquefaction plant components.