The grown-in oxygen precipitates in conventional Czochralski (CZ) silicon and nitrogen-doped Czochralski (NCZ) silicon have been investigated by means of transmission electron microscopy (TEM). Tiny oxygen precipitates about 5nm in size were observed in the NCZ specimens. It is believed that the oxygen precipitates may have grown from the heterogeneous nuclei of nitrogen-related complexes formed at a low temperature of 650℃.