A method for fabricating semiconductor device includes the steps of: forming a semiconductor layer on a substrate; removing part of the semiconductor layer and part of the substrate to form a trench; forming a liner in the trench; removing part of the liner to form a spacer adjacent to two sides of the trench; forming a conductive layer in the trench; forming a metal layer on the conductive layer; forming a mask layer on the metal layer; and patterning the mask layer, the metal layer, and the conductive layer to form a bit line structure.