The formation behavior of grown-in defects which are considered to be oxygen precipitates formed during CZ-Si crystal growth, was investigated by means of infrared light scattering tomography. The following results were obtained. (i) The density of the IR light scattering defects decreases with a reduction in the crystal pulling rate. (ii) The defects are not formed just after solidification, but they grow to a size detectable by LST during cooling to about 1100 degrees C. (iii) The defect density decreases by slow cooling in the temperature range from to 1500 to 1000 degrees C, while their size increases. The formation mechanism of the defects was qualitatively discussed from the point of view of the interaction between oxygen atoms and point defects by a consideration of the free energy change and the critical radii of nuclei for oxygen precipitation. It was suggested that the formation of the defects depends on the vacancy concentration. In the case of a constant vacancy concentration, their density and size are determined by the cooling rate in the temperature range from 1150 to 1000 degrees C.