There are a method and an apparatus for modifying a graphene, and more specifically, a method and an apparatus for modifying a graphene capable of obtaining the graphene having a desired crystallite size by repeating a process for modifying the graphene using subcritical or supercritical carbon dioxide several times. According to the method and the apparatus for modifying the graphene of the present invention, the graphene having excellent electrical conductivity and dispersibility may be obtained.