Nickel sulfide (NiS) hollow spheres have been successfully synthesized by gamma-irradiation, at room temperature, of an aqueous PMMA-CS2-ethanol solution that contains NiSO4-6H(2)O. Electron microscopy results show that the diameter of the NiS hollow spheres and the thickness of the sphere shells are about 500 nm and 20 nm, respectively. The room-temperature UV-vis absorption spectrum of the NiS hollow spheres gives a peak centered at around 233 nm (5.56 eV) with a remarkable blue-shift relative to that of bulk NiS (2.1 eV). This remarkable blue-shift may be attributed to the small dimensions of the materials. A possible growth mechanism of NiS hollow spheres by gamma-irradiation method is also presented. The successful preparation of NiS hollow spheres on a large scale under mild conditions could be of interest for both applications and fundamental studies.