The vaporization of liquid or boil-off-gas (BOG), which is caused by the heat absorption from the surroundings is one of the main issues in treating a liquefied gas (LNG) storage tank. In the offshore, sloshing can be occurred in the LNG storage tank and lead convective heat transfer in the tank. The generation of BOG is directly related to the convective heat transfer in the storage tank. In this work, the evaporative heat rate from the liquid-vapor interface in a storage tank is numerically investigated by solving the conservation equations for mass momentum and energy with a commercial CFD software FLUENT. The VOF method is used for tracking the liquid-gas interface. The effect of sloshing of a partially filled containner on convection and evaporation heat transfer in the storage tank is investigated.