Slow positron beam and coincidence Doppler broadening techniques have been used to follow temperature-induced defects and structural changes in Cz-Si with an initial oxygen content of 1.1×1018 cm-3. Oxygen is recognized as a peak at about 11.85×10-3m0c on the ratio curves. For Cz-Si annealed at 480 oC/15h or 600 oC/15h, the ratio curves show the presence of vacancy-like defects, but they are not associated with oxygen. For Cz-Si annealed at 480 oC/15h, then followed by a 600 oC/15h heat treatment, the ratio curves show the signal of O atom. The ratio curves of Cz-Si, thermally treated by a two-step (480oC/15h + 600oC/15h) pre-annealing, followed by a one-step annealing under different hydrostatic argon pressures and annealed temperatures, also show a peak at 11.85×10-3m0c. The height of the peak varies with different samples. Experimental results indicate that the SiO2 film will form on the surface of Cz-Si after the heat treatment.