A thin film transistor (TFT) having improved characteristics, a method for fabricating the same, and an organic light emitting display device (OLED) including the same. The TFT is constructed with a substrate, a semiconductor layer disposed on the substrate and including a channel region, source and drain regions, a gate insulating layer disposed on the semiconductor layer, a gate electrode disposed on the gate insulating layer and corresponding to the channel region, an interlayer insulating layer disposed on the gate electrode, and source and drain electrodes electrically connected to the source and drain regions of the semiconductor layer. The channel region is made from polycrystalline silicon (poly-Si), and the source and drain regions are made from amorphous silicon (a-Si). The polycrystalline silicon of the channel region is formed by crystallizing amorphous silicon using Joule's heat generated by the gate electrode.