A method for fabricating a bottom electrode is provided. In this method a dielectric layer is formed on a substrate having a source/drain region. A via hole is formed in the dielectric layer to expose the source/drain region. A patterned, doped polysilicon layer is formed on the dielectric layer and fills the via hole, wherein the cross-section of the patterned doped polysilicon layer is arced or polygonal. The surface of the patterned polysilicon layer is transformed into an amorphous silicon layer. A hemispherical-grain layer is formed on the amorphous silicon layer.