An LNG storage tank and a method of treating boil-off gas using the same are disclosed. The LNG storage tank and the method of treating boil-off gas enable the pressure of the LNG storage tank to be maintained in a stable state without consuming boil-off gas generated in the LNG tank as propulsion fuel or without re-liquefying the boil-off gas. The storage tank comprises a thermal insulation wall, and has strengthened structure to permit a pressure increase resulting from heat influx to the LNG tank and boil-off gas generation. The method comprises permitting a pressure increase by the boil-off gas generation and LNG cargo temperature increase in the storage tank without treating the generated boil-off gas in the storage tank, so that the boil-off gas is accumulated without extracting or loss in the storage tank.