Post‐growth patterning to <15 &mgr;m×15 &mgr;m size patterns combined with thermal annealing at 850 °C for ≳15 min eliminates the dark line defects (DLDs) in GaAs‐on‐Si as shown by the spatially resolved photoluminescence technique. Patterning to small size islands of GaAs facilitates dislocation migration laterally out of the crystal, and thermal annealing provides the activation energy for the dislocations to migrate and interact. Patterning to small size features also significantly reduces the thermally induced biaxial tensile stress as reported earlier. On large size patterns, the density of DLDs is significantly reduced near the surface leaving larger volume of the material free from DLDs.