A fabrication method of a thin film transistor includes providing a substrate at first. Thereafter, a first gate is formed on the substrate. An insulator is then formed to cover the first gate and a portion of the substrate. After that, a channel structure is formed on the insulator above the first gate. In addition, a metal layer is formed to cover the channel structure and a portion of the insulator. Next, the metal layer is patterned, and at least the metal layer on two sidewalls of the channel structure is retained to form a source and a drain, respectively. Moreover, a passivation layer is formed to at least cover the source, the drain and a portion of the insulator.