A flat panel display device with an oxide thin film transistor is disclosed which includes: a buffer film formed on a substrate; an oxide semiconductor layer which has a width of a first length and is formed on the buffer film; a gate insulation film which has a width of a second length and is formed on the oxide semiconductor layer; a gate electrode which has a width of a third length and is formed on the gate insulation film; an interlayer insulation film formed on the entire surface of the substrate provided with the gate electrode; source and drain electrodes formed on the interlayer insulation film and connected to the oxide semiconductor layer; a passivation film formed on the entire surface of the substrate provided with the source and drain electrode; and a pixel electrode formed on the passivation film and connected to the drain electrode. The first length is larger than the second length and the second length is larger than the third length.