The phase diagram in nm-sized Au-Sb alloy system has been studied by transmission electron microscopy. With decreasing system size, the increase of solid solubilities in solid solution or intermetallic compounds are significantly enhanced. With a further decrease in the system size. the pure substance continuously changes into solid solution, a highly disordered ultra-fine grain nanostructure aud then to intermetallic compound as the solute concentration increases. The formation of the highly disordered ultra-fine grain nanostructuresmay be related to the low heterogeneous nucleation probability. (C) 2002 Elsevier Science B.V. All rights reserved.