Abstract: Computation of evaporation and holding time of Liquefied Natural Gas (LNG)tank with marine fuel is important in the design of boil-off gas treatment system for the LNG powered ships.A computation method for the evapo-ration and holding time of LNG tank with marine fuel is presented.The evaporation rate of liquid nitrogen and LNG is an-alyzed respectively by using the heat transfer formula.The relationship between static evaporation rate of liquid nitrogen and LNG is derived.For the situations of small pressure variations,the computation formula of LNG evaporation rate is obtained by the relationship of density of LNG and its latent heat of vaporization under different pressures.The computa-tion formula of LNG evaporation is therefore deduced from the average evaporation rate of LNG.The holding time of LNG tank is computed according to the change of gas weight in the storage tank.As a case study,the evaporation and holding time of a 20 m3 LNG tank are computed by two different methods.The evaporation computed with average evapo-ration rate of LNG is 20.72 kg.It is 34.1 kg in computation with evaporation rate of liquid nitrogen.The holding time computed by the former method is 21.64 d;and 13.15 d of the later method.The evaporation computed with average e-vaporation rate of LNG is 13.38 kg less,and the holding time is 8.49 d shorter.The temporal variations of the evaporation computed by these two methods are generally consistent,while the evaporation and the holding time show great differ-ences.Comparing with the method based on the evaporation rate of liquid nitrogen,this method using average evaporation rate of LNG,provides solutions more consistent with the observations.This method can be further applied to guide the design of boil-off gas treatment system for the LNG powered ships.