A simple and robust route is described to the synthesis of single-crystal Au nanospheres with diameters controlled in the range of 5 nm to 150 nm. The success of this synthesis relies on the use of single-crystal Au spheres with different diameters as the seeds for successive growth and the use of a slow injection rate for the precursor to enable surface diffusion for the atoms added onto the surface of a seed. The diameters could be precisely controlled by varying the size and/or number of the seeds. The products exhibit excellent uniformity in terms of both size and shape and they are expected to find widespread use in a number of applications, including self-assembly, fabrication of metallodielectric photonic crystals, plasmonics, and biomedical research.