We report a rational approach to fabricate graphene nanoribbons (GNRs) with sub-10 nm width by employing chemically synthesized nanowires as the physical protection mask in oxygen plasma etch. Atomic force microscopy study shows that the patterns of the resulted nanoribbons replicate exactly those of mask nanowires so that ribbons or branched or crossed graphene nanostructures can be produced. Our study shows a linear scaling relation between the resulted GNR widths and mask nanowire diameters with variable slopes for different etching times. GNRs with controllable widths down to 6 nm have been demonstrated. We have fabricated GNR field effect transistors (FETs) with nanoribbons directly connected to bulk graphene electrodes. Electrical measurements on an 8 nm GNR-FET show room temperature transistor behavior with an on/off ratio around 160, indicating appreciable band gaps arise due to lateral confinement. We find the on/off ratio in the log scale inversely scales with ribbon width. This approach opens a new avenue to graphene nanoribbons and other graphene nanostructures in the deep nanometer regime without sophisticated lithography. It thus opens exciting new opportunities for graphene nanodevice engineering.