We have addressed essential problems for the determination of the size distribution and volume density of crystal defects by bright field infrared laser interferometer [optical precipitate profiler (OPP)]. By taking into consideration the influences of the intensity profile and phase shift of the incident laser beam along the optical axis, we developed a new statistical data processing method to determine the true size distribution and volume density of the crystal defects from the raw OPP data. Using this method we evaluated the size distribution and volume density of oxygen precipitates introduced in a CZ–Si wafer and the validity of the new method was confirmed by comparing the obtained results with those obtained by the etching method and transmission electron microscopy. ©1998 American Institute of Physics.