This article demonstrates, from the theoretical point of view, that owing to the phonon-assisted relaxation rate of the excited electron-hole pair’s transiting to lower states decreases as the scale of Si particle is reduced, the optically excited electron-hole pair in the nanometer silicon particle with sufficient small scale generally tunnel into theSiOxlayers which enclose the Si particles and recombine radiatively through the luminescence centers there to emit visible light rather than recombine radiatively in the Si particles. It is proved also that when the density of the luminescence center is2.5×1022/m3,the upper limit of the average scale of the nanometer silicon particles in the ensemble capable of emitting red light in room temperature is around 8.7 nm, which is much bigger than the generally estimated value of 3 nm. ©1997 American Institute of Physics.