A fabrication method of under bump metallurgy (UBM) structure is provided. A blocking layer is applied over a surface of a semiconductor element formed with at least one bond pad and a passivation layer thereon. The passivation layer covers the semiconductor element and exposes the bond pad, and the blocking layer covers the bond pad and the passivation layer. The blocking layer is formed with at least one opening at a position corresponding to the bond pad. Metallic layers are formed on a surface of the blocking layer and at the opening. The metallic layers are patterned to form a UBM structure at the opening corresponding to the bond pad. Then the blocking layer is removed. The blocking layer can separate the metallic layers for forming the UBM structure from the passivation layer to prevent metallic residues of the UBM structure from being left on the passivation layer.