Ytterbium-implanted Si(001) wafers annealed at different temperatures (800, 900, 1000 and 1200 degrees C) have been examined by means of cross-section transmission electron microscopy (XTEM) and electron diffraction. The results indicate that two layers of defects exist in the samples, one mainly includes microtwins and the other includes ytterbium precipitates. The distribution and morphology of the defects depend mostly on the thermal annealing : the higher the annealing temperature, the larger the size of the defects and the closer are the layers of defects to the wafer surface. High-resolution images show the different characteristics and details of the defects under various annealing conditions. The relation between microstructure and luminescence response is also discussed.