In order to correlate the size, numerical concentration, and size distribution of free volumes with the nanostructure of crosslinked polysiloxane (cured MQ resin), free volumes of polysiloxanes of different degree of crosslinkage polysiloxanes were observed by positron annihilation lifetime measurements in the temperature range from 30 K to room temperature. The free-volume diameters of the highly crosslinked polysiloxane were found to be larger in the temperature range studied and their distributions become broader above T-g (150 K) than those for the lowly crosslinked polysiloxane, whereas free volume contents (number density) were lower for the highly crosslinked polysiloxane. The results suggest the heterogeneous and distorted nanostructure of prepolymers in the highly crosslinked polysiloxane.