Shapes and sizes of Au nanocrystals prepared by reducing HAuCl4 in ethylene glycol (EG) depended strongly on the presence of O-2 in EG. At 110 degrees C icosahedrons, one-dimensional (1-D) rods and wires, and spherical particles were major products under bubbling N-2 gas, whereas octahedrons and nanoplates were dominant products under bubbling O-2 gas. At 198 degrees C decahedrons and icosahedrons were major products under bubbling N-2 gas, whereas the yields of larger octahedrons, nanoplates, and spherical particles increased under bubbling O-2 gas. These results show that bubbling N-2 or O-2 gas at different temperatures can be used as a new experimental parameter for the shape-and size-controlled synthesis of Au nanostructures.