Single-crystal gold nanoshects, with triangular, hexagonal, or truncated triangular shapes, from several to tens of micrometers across and tens of nanometers thick, have been successfully synthesized in high yield via a simple and low-cost chemical route in an ethylene glycol solution, on the basis of a polyol process. The planar surfaces of the Au nanosheets are atomically flat and correspond to {111} planes; the lateral surfaces are {110} planes. The nanosheets show strong optical absorption in the near infrared region of the electromagnetic spectrum. Both the ethylene glycol and the surfactant polyvinylpyrrolidone (PVP), in the solution play important roles in the formation of the Au nanosheets. The concentrations of the precursors (PVP, HAuCl4) and the reaction temperature are also crucial to the morphology and size of the final product. The formation of such large, single-crystal nanosheets is explained by the preferential adsorption of some species of molecules from the solution onto the {111} planes of Au nuclei, and the connection of small, triangular nanosheets. These nanosheets could be used easily, for example, in gas sensors, in the fabrication of nanodevices and substrate materials, in property studies, and also for inducing hypothermia in tumors.