The nanostructure of GaAs nanocrystals grown on Si(1 0 0) surface was investigated by plan-view and cross-sectional transmission electron microscopic observations. The shape change from nearly isotropic base to anisotropic rectangular base elongated along [0 1 1] or [0 1 1] direction appears pronouncedly in GaAs nanocrystals above the major axis length of around 20nm. In the GaAs nanocrystals with anisotropic rectangular base, there are parallel more fringes along the major axis and dense stacking faults along the minor axis, we found that {0 2 2} lattice spacings along the minor and major axes of the nanocrystal are nearly equivalent to those in bulk Si and GaAs. respectively. These results suggest that the stacking faults are formed on one of four {1 1 1} slip planes of larger-sized GaAs nanocrystal to relax the accumulated lattice strain, and that the GaAs nanocrystal preferentially grows along the <0 1 1> direction in which the stacking faults are repeatedly formed. (C) 2009 Elsevier B.V. All rights reserved.