Temperature and size dependence of photoluminescence (PL) of nano-silicon embedded in SiO2 matrix samples were studied. In these measurements, four samples with deferent implantation dose showed their similar tendency. Their nano-structure was investigated by high resolution electron microscopy (HREM) and selected area diffraction (SAD) which confirmed that only those prepared with the higher Si implantation dose formed crystal nano-silicon. The further analysis of the dependence of thermal activation energy on the PL emission energy demonstrates their different behaviors. This leads to an optical method to detect the crystal nano-silicon through comparing their thermal activation energy with the Calcott model, a model that implies the emission from nanocrystallites. Furthermore, the appearance of thermal activation energy in amorphous nano-silicon is discussed in the light of recombination mode of localized carriers through the band-tail state. (C) 2008 Elsevier Ltd. All rights reserved.