Disclosed herein is an organic thin film transistor comprising a substrate, a gate electrode, a gate insulating layer, an organic semiconductor layer, source-drain electrodes and a protective layer wherein a buffer layer is interposed between the organic semiconductor layer and the protective layer. Such a transistor minimizes the deterioration in the performance of the transistor due to ambient air containing oxygen and moisture, and the degeneration in the performance of the transistor caused during mounting a display device.