It has been shown that a two-peak green photoluminescence (PL) of porous silicon (PS) can be obtained from a continuous blueshift of a red PL by positron irradiation. At room temperature, PS samples were irradiated in air by energetic positrons coming from the conventionally used isotope22Na(∼20 &mgr;Ci).With increasing positron irradiation time, an originally red PL shifted continuously to green, then a two-peak PL appeared with a weak high-energy emission band (529 nm) and a low-energy dominant band (562 nm). The intensity of this high-energy band was enhanced by prolonged positron irradiation. The electron-spin-resonance signal combined with infrared absorption showed that positron irradiation created dangling bonds and stimulated oxide growth in PS. An interpretation is given on the basis of quantum confinement and atomiclike nature for very small nano-Si crystallites. ©1997 American Institute of Physics.