Disclosed is a system for treating boil-off gas of a ship having a storage tank storing liquefied natural gas; and an engine using the liquefied natural gas stored in the storage tank as fuel. The system for treating boil-off gas of a ship includes a first stream of boil-off gas discharged from the storage tank after being generated from the liquefied natural gas in the storage tank; a second stream of boil-off gas branched from the first stream to be supplied to the engine as fuel when the amount of the first stream is greater than the amount of fuel required by the engine; and a third stream of boil-off gas not supplied to the engine among the first stream. The third stream is liquefied by heat exchange with the first stream in a heat exchanger, thereby treating boil-off gas without using a reliquefying device having a separate freezing cycle.
