The recent increase in marine transportation of Liquefied Natural Gas (LNG) made the design of modern LNG vessels more demanding in terms of both increased cargo capacity and fuel economy. A detailed thermoeconomic model of the energy system of a LNG vessel is developed in this study. The model addresses both the energy system components and the production of boil-off gas, which is the main fuel of these vessels. A generic energy system that comprises various technology and configuration options is developed. In addition, a realistic operating profile of a LNG vessel is considered. The thermoeconomic model is subsequently used for synthesis, design and operation optimization of the system.An initial version of this paper was published in June 2007 in the proceedings of the ECOS'07 Conference, Padova, Italy.