In this work, a coplanar waveguide employing chemically modified graphene was fabricated on a polyethylene terephthalate (PET) substrate and its RF loss characteristics were investigated. According to the results, the coplanar waveguide showed much lower loss characteristics than conventional graphene-based transmission line. The insertion loss of conventional graphene/Si structure was - 24 to - 33 dB at 1 GHz, but that of the graphene/PET structure was - 0.773 dB at the same frequency. However, the graphene/PET structure showed a comparatively higher loss than the gold - Ti/PES structure. The higher loss of the graphene/PET structure originated from the comparatively lower conductivity of graphene. The results of the present study indicate that the graphene/PET structure is prospective for application in flexible RF devices.