A method of manufacturing a flexible display device includes forming a graphene adhesive layer on a carrier substrate, forming a flexible substrate on the graphene adhesive layer, forming a first barrier layer on the flexible substrate, forming a display element part on the first barrier layer, forming a protective film on the display element part, separating the flexible substrate from the carrier substrate, removing a remaining portion of the graphene adhesive layer from a surface of the flexible substrate, and forming a second barrier layer on the surface of the flexible substrate, after removing the remaining portion of the graphene adhesive layer from the surface of the flexible substrate.