Disclosed is a method for preparing a reduced graphene oxide-magnesium nanocrystal composite. The method includes contacting graphene oxide with a first reducing agent to prepare a reduced graphene oxide, and co-reducing the reduced graphene oxide and a precursor of magnesium in the presence of a second reducing agent to prepare a reduced graphene oxide-magnesium nanocrystal composite, wherein by adjusting the amount of the first reducing agent in contact with the graphene oxide, the size of the magnesium nanocrystals in the composite may be controlled.