A metal oxide semiconductor transistor includes a gate, a metal oxide active layer, a gate insulating layer, a source, and a drain. The metal oxide active layer has a first surface and a second surface, and the first surface faces to the gate. The gate insulating layer is disposed between the gate and the metal oxide active layer. The source and the drain are respectively connected to the metal oxide active layer. The second surface defines a mobility enhancing region between the source and the drain. An oxygen content of the metal oxide active layer in the mobility enhancing region is less than an oxygen content of the metal oxide active layer in the region outside the mobility enhancing region. The metal oxide semiconductor transistor has high carrier mobility.