A number of studies have been presented in regard with the economy of LNG carrier. This paper, firstly, discusses the relation between the economy of a conventional LNG carrier driven by steam turbine engine and the different boil off gas rates. And, further, an economical feasibility of the future diesel engine driven LNG carrier, which installs the reliquefaction plant on board ship to liquefy the boil off gas, is discussed herewith, taking into account of the practical operation pattern of the plant, and proposes some break even point of the investment to the reliquefaction plant.