Abstract: A thin film transistor and a manufacturing method thereof are provided. The thin film transistor includes a gate, an oxide channel layer, a gate insulating layer, a source, a drain and a dielectric layer. The gate is disposed on a substrate. The oxide channel layer, disposed on the substrate, is stacked with the gate. A material of the oxide channel layer includes a metal element. The metal element content shows a gradient distribution along a thickness direction of the oxide channel layer. The gate insulation layer is disposed between the gate and the oxide channel layer. The source and the drain are disposed in parallel to each other, and connected to the oxide channel layer. Sides of the source and the drain, facing away from the substrate, are covered by the dielectric layer.