As new type of vehicle fuel, Liquefied Natural Gas (LNG) has been widely used increasingly all over the world and LNG plants are under construction or plan in many locations of China. A large amount of boil off gas (BOG) will be generated from LNG production and storage and accurate calculation of BOG volume in different working conditions and selection of the optimal treatment process are very important for reducing energy consumption and improving economic benefit of LNG plant. In combination with technological process of LNG plant, applies is the theory analysis method combined with engineering practice experience to analyze BOG generation principle and discuss BOG treatment process in LNG plant and proposed is a systematic method for calculating BOG volume generated during LNG production and storage in LNG plant.