We report strong visible photoluminescence (PL) at room temperature from oxidized Si nanometer-sized spheres with a spherical crystalline Si (c-Si) core and an amorphous SiO2 (a-SiO2) surface layer. The peak energy of the broad PL spectrum is about 1.65 eV, which is independent of the core diameter. We propose a model in which excitons are confined on a spherical shell, an interfacial layer between the c-Si core and the a-SiO2 surface layer, and in which the exciton confinement enhances the oscillator strength and the PL intensity. © 1993 The American Physical Society.