The invention provides an improved thin film transistor (TFT) that can be formed at room temperature and has an improved contact resistance between an active layer and source and drain electrodes, and further provides a flat display device using such a TFT. The TFT includes an active layer including at least two nano particle layers which include at least one nano particle type, an insulating layer interposed between the nano particle layers, a gate electrode insulated from the active layer, and source and drain electrodes formed in respective channels, the source and drain electrodes contact one of the nano particle layers of the active layer. The structure of the TFT facilitates the simultaneous manufacturing of a plurality of different types of TFTs.