The present invention provides a thin film transistor and a method of fabricating the thin film transistor, an array substrate and a method of fabricating the array substrate, and a display device. The thin film transistor includes a substrate and a gate, an insulation layer, an active layer, a source and a drain which are provided on the substrate. A spacer layer is also provided between the gate and the active layer, and the spacer layer overlaps at least with one of the gate and the active layer having a smaller area in an orthographic projection direction. The spacer layer can effectively prevent material forming the gate from being diffused into the active layer, thereby ensuring stability of performance of the thin film transistor. In the array substrate utilizing the thin film transistor, the spacer layer further extends to a region corresponding to a gate line.