An organic thin film transistor in which source and drain electrodes have a double layer structure to aid patterning of an organic semiconductor layer using a laser beam, and a flat display device having the organic thin film transistor. The organic thin film transistor includes: a gate electrode; a source electrode and a drain electrode insulated from the gate electrode; an organic semiconductor layer insulated from the gate electrode and having a portion patterned to electrically connect to the source and drain electrodes; and a protection layer formed on the source and drain electrodes.