The structural properties of an ultrathin (3 nm) Si layer sandwiched between two thin SiO 2 layers subjected to thermal annealing have been investigated by energy filtered transmission electron microscopy (EFTEM). It has been demonstrated that the first stages of the thermal evolution of the Si layer involve the formation of a highly interconnected Si network, followed by the appearance of well defined nanoclusters (both amorphous and crystalline). The quantitative analysis of the EFTEM data allowed determination of the size and density of the Si nanoclusters, as well as their crystalline fraction. This information has been used to explain the dependence of the system photoluminescence on the annealing temperature.