Based on the statistical theory of X-ray dynamical diffraction, a method to obtain the static Debye-Waller facter from section topographs is described. By analyzing the intensity distribution of the Pendelldsung fringes in the X-ray diffraction topograghs for heat treated CZ and MCZ silicon single crystals, the number density and the size of the oxygen precipitates which are smaller than X-ray topographic resolution are estimated. This quantitative analysis method provides a new means of studying the micro-defects.