a method for fabricating a thin film transistor is provided . first , a gate is formed on a substrate . a gate - insulating layer is formed to cover the gate . a patterned semiconductor layer is formed on the gate - insulating layer . a first and a second conductive layer are formed on the patterned semiconductor layer in sequence . the second conductive layer is patterned such that each side of thereof above the gate has a taper profile and the first conductive layer is exposed . a first plasma process is performed to transform the surface and the taper profile of the second conductive layer into a first protection layer . the first conductive layer not covered by the first protection layer and the second conductive layer is removed to form a source / drain . the source / drain is with fine dimensions and the diffusion of metallic ions from the second conductive layer to the patterned semiconductor layer can be avoided .