Provided are an organic thin film transistor, a flat panel display device and methods of manufacturing these. The organic thin film transistor includes: source and drain electrodes and an organic semiconductor layer formed on a surface of a substrate; a gate electrode insulated from the source and drain electrodes and the organic semiconductor layer; wherein a thickness of at least a portion of the gate insulator above both the source and drain electrodes is larger than a thickness of at least a portion of the gate insulator above the channel region of the organic semiconductor layer.