The present paper is concerned to the thermal analysis during the cool-down period of 138,000 m$^3$class GTT MARK-III membrane type LNG carrier servicing with LNG from the Middle East to Korea. It is the cool-down period that cools the insulation wall and the gas in LNG tank to avoid the thermal shock as the start of loading of -162$^{circ}C$ LNG. For six hours of the standard cool-down period, the temperature of NG falls down from -4$0^{circ}C$ to -13$0^{circ}C$ and especially the mean temperature of the 1st barrier in the top side insulation wall falls down from -38.38$^{circ}C$ to -122.42$^{circ}C$ in case of IMO design condition. By the 3-D numerical calculation about the cargo tank and the cofferdam, the temperature variation in hulls and insulations is precisely predicted in this paper. And the mean temperature variation of gas is calculated as the function of the spraying rate by the heat balance model during the cool-down period.