Transparent, flexible carbon-based pentacene field-effect transistors (FETs) were successfully fabricated from monolayer graphene electrodes on plastic substrates. One-atom-thick monolayer graphene provides an ideal material for source/drain electrodes for efficient charge injection and transport, resulting in low contact resistance between the electrodes and the pentacene films. Thus, pentacene FETs with patterned graphene electrodes exhibit significantly higher performances than those of common metal electrodes.