GeSi alloy nanocrystals embedded in amorphous SiO2 matrix were fabricated via magnetron sputtering method and subsequent high-temperature annealing. The crystallite morphology and microstructural defects of the alloy nanocrystals were characterized by high-resolution transmission electron microscope observations. It is observed that GeSi nanocrystals with an average diameter of similar to 10 nm are mostly spherical and have low defect density. Such nanocrystals with twinning configurations, including single-twinning, multi-layer twinning, and five-fold twinning, were observed in similar to 9% of total nanocrystals. The fabrication of spherical GeSi nanocrystals with low defect density is important for further studies in photoluminescence and acoustic phonon properties of the GeSi alloy nanocrystals. (c) 2006 Elsevier B.V. All rights reserved.