We report a seed-mediated synthesis method for the preparation of gold nanocrystals with systematic shape evolution from truncated cubic to cubic, trisoctahedral, and rhombic dodecahedral structures in aqueous solution for the first time. Nanocrystals with transitional morphologies were also synthesized. The combination of using cetyltrimethylammonium chloride (CTAC) surfactant and a very small amount of NaBr to control the bromide concentration in the growth solution was found to be critical to the formation of nanocubes. Variation in the volume of ascorbic acid added to the growth solution enabled the fine control of nanocrystal morphology. Nanocubes and rhombic dodecahedra with controlled sizes of 30-75 nm were prepared by adjusting the volume of the seed solution added to the growth solution. They can self-assemble into ordered packing structures on substrates because of their uniform sizes. XRD, TEM, and UV-vis absorption characterization of the different products synthesized have been performed. By increasing the bromide concentration 5-fold that used to make the nanocubes, unusual right bipyramids of gold bounded by six {100} faces were produced. The high product purity and excellent size control of this facile synthetic approach should make these novel gold nanostructures be readily available for a wide range of studies.