In this letter, we address the effectiveness of patterned growth in reducing defect density in GaAs on Si. Defect reduction is considered to be subject to the effect of edge profile and the density of defects in initial GaAs islands. Pattern edges (mask edges or large steps) are often the sites for generating microtwins/stacking faults. Mutual interaction of dislocations inhibits the gliding of dislocations to pattern edges, thus affecting the effectiveness of patterned growth in improving the epilayer quality. The study also shows that surface orientation influences the formation of misfit dislocations with a preferred Burgers vector. This observation is interpreted by considering the variation of misfit with different lattice planes across the interface between Si and ‘‘tilted’’ GaAs lattices.