A method of fabricating a vertical thin film transistor (vertical TFT) is disclosed, wherein a shadow mask is used to fabricate the TFT device in vertical structure. First, a metal layer is formed, which serves as ribs and a gate layer. Next, a shadow mask is disposed on the gate layer. Afterwards, the shadow mask is used as a mask to form a source layer, an organic semiconductor layer and a drain layer. Thus, the process is simplified. Since no photolithography process is required, and therefore damage of the organic semiconductor layer is avoided and a vertical TFT with desired electrical characteristics may be obtained.