A novel phosphine-protected Au-20 nanocluster was isolated through the reduction of Au(PPhpy(2)) Cl by NaBH4 (PPhpy(2) = bis(2-pyridyl)-phenylphosphine). Its composition was determined to be [Au-20(PPhpy(2))(10)Cl-4]Cl-2, and single crystal X-ray structural analysis revealed that the Au-20 core can be viewed as being generated from the fusion of two Au-11 clusters via sharing two vertices. Optical absorption spectroscopy indicated this Au-20 has a large HOMO-LUMO gap (E-g approximate to 2.24 eV). This is the first example of a ligand-protected gold nanocluster with a core generated from incomplete icosahedral Au-11 building units.