A vertical transistor device and its fabrication method are provided. The vertical transistor device includes a semiconductor substrate, first sources/drains and second sources/drains. The semiconductor substrate includes a bottom portion and a fin portion. The fin portion is located on the bottom portion. The fin portion includes an upper portion and a lower portion located between the bottom portion of the semiconductor substrate and the upper portion. The lower portion includes a narrow portion having a width smaller than a width of the upper portion, and the narrow portion contacts an interface portion of the upper portion. The sources/drains are disposed on the on the narrow portion of the lower portion of the fin portion. In the method for fabricating the vertical transistor device, the lower portions of the fin portions are patterned to form the narrow portions where the sources are disposed.