A method for forming a thin film transistor on a substrate is disclosed. A gate electrode and a gate insulation layer are disposed on a surface of the substrate. A deposition process is performed by utilizing hydrogen diluted silane to form a silicon-contained thin film on the gate insulation layer first. A hydrogen plasma etching process is thereafter performed. The deposition process and the etching process are repeated for at least one time to form an interface layer. Finally, an amorphous silicon layer, n+ doped Si layers, a source electrode, and a drain electrode are formed on the interface layer.