A thin film transistor (TFT) and a fabricating method thereof are provided. The TFT includes a channel layer, an ohmic contact layer, a dielectric layer, a source, a drain, a gate, and a gate insulating layer. The channel layer has an upper surface and a sidewall. The ohmic contact layer is disposed on a portion of the upper surface of the channel layer. The dielectric layer is disposed on the sidewall of the channel layer, and does not overlap with the ohmic contact layer. The source and the drain are disposed on portions of the ohmic contact layer and the dielectric layer. A portion of dielectric layer is not covered by the source or the drain. The gate is above or below the channel layer. The gate insulating layer is disposed between the gate and the channel layer.