The gamma-radiation method has been firstly used to prepare nanocrystalline powders of cuprous oxide from cupric sulfate aqueous solution in the presence of CH3COOH/CH3COONa buffer pair. X-ray powder diffraction reveals that the product is single phase of cuprous oxide. Electron microscopy shows that the average particle size of cuprous oxide prepared is about 14 nm. Influences of experimental conditions on particle size of cuprous oxide and the mechanism of the formation of cuprous oxide are discussed.