For liquid ethylene gas (LEG) vessels, developing high-efficient re-liquefaction plant to handle the continuously yielded boil-off gas (BOG) could significantly improve its energy efficiency. In this study, an ejector enhanced re-liquefaction process (EERP) for LEG vessels is proposed to improve the performance of the conventional re-liquefaction process (CRP) with a cascaded two-stage compression refrigeration cycle. In Aspen HYSYS, an ejector model was developed to analyse the performance of the proposed cycle system. The effects of the evaporating pressure of upper cycle p(R0) and the intermediate pressures of upper and bottom cycles (p(R3) and p(E3)) on the performances of both the CRP and EERP were studied. The results show that better performances can be reached with lower p(R0) for the CRP and EERP. In order to achieve the best performances, the optimal p(R3) and p(E3) are, respectively, lifted to 510 kPa and 630 kPa by the EERP. When the proposed cycle system with the EERP is employed to re-liquefy 3000 kg/h of ethylene BOG, its coefficient of performance (COP) and exergy efficiency can be improved by 2.95-5.31% and 2.70-4.86% over that of the cycle system with the CRP. Correspondingly, the EERP could reduce the total power consumption of 15.7-27.9kW compared to the CRP. (C) 2016 John Wiley & Sons, Ltd.