The present invention relates to a through silicon via (TSV). The TSV is disposed in a substrate including a via opening penetrating through a first surface and a second surface of the substrate. The TSV includes an insulation layer, a barrier layer, a buffer layer and a conductive electrode. The insulation layer is disposed on the surface of the via opening. The barrier layer is disposed on the surface of the insulation layer. The conductive electrode is disposed on the surface of the buffer layer and fills the via opening. The buffer layer further covers a surface of the conductive electrode at the side of the second surface. The present invention further discloses a method of forming the TSV.