We have studied the nucleation and propagation of threading dislocations in GaAs on Si epitaxial layers, and have found several techniques which are effective in reducing their density. The use of substrates properly tilted off (100) reduces the dislocation density as the presence of steps helps create perfect edge dislocations with their Burgers vector parallel to the interface and thus do not propagate into the bulk epitaxial layer. Cross sections by transmission electron microscopy show that the incorporation of an InGaAs/GaAs strained‐layer superlattice reduces the density of threading dislocations above it by a factor of 10, clearly demonstrating the effectiveness of this technique. These methods lead to a dislocation density of 103cm−2near the surface of 2 &mgr;m layers which is five orders of magnitude lower than what has been obtained previously. We have also found that the density of oval defects is much lower for GaAs on Si than for GaAs on GaAs.