The larger size gold nanoparticles typically adopt a face-centered cubic (fcc) atomic packing, while-in the ultrasmall nanoclusters the packing styles of Au atoms are diverse, including fcc, hexagonal close packing (hop), and body-centered cubic (bcc), depending on the ligand protection. The possible conversion between these packing-structures is largely unknown: Herein, we report the growth of a new Au-21(S-Adm)(15): nanocluster (S-Adm = adamantanethiolate) from Au-18(SR)(14) (SR = cyclohexylthiol), with the total structure determined by X-ray crystallography. It is discovered that the hcp Au-9-core in Au-18(SR)(14) is transformed to a fcc Au-10-core in Au-21(S-Adm)(18). Combining with density functional theory (DFT) CalCulations, we provide critical infOrmation about the growth mechanism (geometrical and electronic structure).and the origin of fcc-structure formation for the thiolateprotected gold nanoclusters.