A photomask for fabricating a thin film transistor (TFT) is disclosed. The photomask includes a translucent layer disposed on a transparent substrate and covering U-shaped and rectangular channel-forming regions of the transparent substrate. First and second light-shielding layers are disposed on the translucent layer and located at the outer and inner sides of the U-shaped channel-forming region, respectively, and third and fourth light-shielding layers are disposed on the translucent layer and located at opposite sides of the rectangular channel-forming region, respectively, to serve as source/drain-forming regions. An end of the third light-shielding layer extends to the first light-shielding layer. A plurality of first light-shielding islands is disposed on the translucent layer and located within the rectangular channel-forming region. A method for fabricating source/drain electrodes of a TFT is also disclosed.