Commercial Czochralski-grown silicon (Cz-Si) and float-zone silicon(Fz-Si) wafers were irradiated with fission neutrons at various fluences from 10^ to 10^ n/cm^2 at temperatures ranging from 473K to 1043K. The irradiation induced defect structures were examined by transmission electron microscopy and ultra high voltage electron microscopy, which were compared with Marlowe code computer simulation results. It was concluded that the vacancy-type damage structure formed at 473K were initiated from collapse of vacancy-rich regions of cascades, while interstitial type defect clusters formed by irradiation above 673K were associated with interstitial oxygen atoms and free interstitials which diffused out of the cascades. Complex defect structures were identified to consist of {113} and {111} planar faults by the parallel beam illumination diffraction analysis
