Silicon substrates were implanted with Si ions at an energy of 60 keV to a dose of 5×1015cm-2followed by a thermal annealing at various temperatures up to 950 oC. Photoluminescence (PL) and infrared absorption (IRA) techniques have been used to characterize these samples. The PL peak positions at 2.07 eV and 1.93 eV undergo redshifts with the increasing annealing temperature. The two IRA peaks at 1080 cm-1and 800 cm-1are ascribed to the Si-O-Si asymmetric stretching and the Si-O bending vibration, respectively. The experimental results indicate that Si nanocrystals embedded in silicon oxide layer can be formed at the annealing temperature 800 oC or higher.