This research investigated the mechanism for the reduction of grown-in defects in Czochralski silicon wafers by high-temperature annealing for various annealing temperatures and ambients. Annealing at 1300 degrees C in an argon ambient could eliminate not only sur face grown-in defects crystal originated particles (COPs), but also bulk grown-in defects at 100 mu m depth from the surface. Annealing at 1200 and 1250 degrees C in an argon ambient also eliminated COPs, but their effects on bulk grown-in defects were limited to shallower regions from the surface with lower annealing temperatures. Oil the other hand, annealing in an oxygen ambient could not reduce grown-in defects even at 1300 degrees C. It is thought that removal of oxide films on the inner walls of grown-in defects is the first step in the reduction process of the grown-in defects. Oxygen annealing cannot remove the oxide film, but rather enlarges them, and the grown-in defects remain. Argon annealing can remove the oxide films, and subsequent absorption of silicon self-interstitials reduces the grown-in defects.