An organic thin film transistor, a flat display device including the same, and a method of manufacturing the organic thin film transistor are disclosed. In one embodiment, the organic thin film transistor includes: i) a substrate, ii) a gate electrode disposed on the substrate, iii) a gate insulation film disposed on the gate electrode, iv) a source electrode and a drain electrode spaced from each other and disposed on the gate insulation film, v) an organic semiconductor layer contacting the source electrode and the drain electrode and having an edge to be distinguished from an adjacent organic thin film transistor, and vi) a cantilever layer disposed to cover the organic semiconductor layer, contacting a portion of a layer which is disposed in or under the organic semiconductor layer, and is exposed to the outside of the edge of the organic semiconductor layer. According to one embodiment, a patterning effect of an organic semiconductor layer is easily obtained and characteristics such as an on/off ratio are improved.