In the seed-mediated growth process of Au nanostructures, the uniformity and structures of the ultrafine seeds determine the purity and structures of the final Au products. Based on the highly shape-dependent surface plasmon resonance properties of Au nanostructures, we present the structures and structural transformation of Au seeds that have overgrown into Au nanostructures. The optical spectra and transmission electron microscopy images of the final product reflected that structures of the ultrafine seed are sensitive to the temperature and surfactants. It is found that the single-twinned Au seeds obtained at 30°C will transform into fivefold-twinned crystals at 90°C, which subsequently grow into Au nanobipyramids with a high surface plasmon resonance quality factor. The introduction of sodium citrate is beneficial for the formation of twinned crystal seeds. Hexadecyltrimethylammonium chloride can be used to keep the size of seeds small enough to allow Ostwald ripening at the initial heat treatment stage.