We report a facile route to form densely packed graphene nanoribbon (GNR) arrays via graphoepitaxial assembly of symmetric P(S-b-MMA). Since guiding channels for graphoepitaxy are the source and drain electrodes in field effect transistor (FET) geometry, we avoid laborious nanopatterning and FET device fabrication processes. By grafting a random copolymer brush on the graphene FET device, perpendicular lamellar domains are aligned normal to the electrode direction, resulting in line arrays connecting the two electrodes. Through optimization of the reactive ion etching conditions, the vertically oriented lamellar domains were transferred to the underlying graphene, leading to GNR arrays that act as conducting channels. This is a simple and efficient fabrication process using the fundamental concepts developed for the graphoepitaxial assembly of symmetric BCPs to create densely packed sub- 20 nm GNR arrays, compared to conventional fabrication process.