We present here a model of the reduction of dislocations in LPB layers by the formation of oriented interfacial dislocations parallel to the surface of the substrate due to the glide motion of substrate threading dislocations which is drived by the thermal stress produced by a temperature difference. In the temprature gradient LPE under a steady natural convection flow, there is a temperature difference across the thickness of the substrte. The temperature difference can cause a thermal shear stress in the fixed substrate. Thick GaAs and Ga1-xAlxAs (x oriented dark line defects. The observation of subface etch pits shows that the epitaxial layers have lower dislocation densities than that of the substrates or are even dislocation-free. The observations of interfacial etch grooves and cathodoluminescence show that substrate threading dislocations bent so that segments run along the interface and a interfacial dislocation network is formed. The transmission electron microscopy observation shows that the majority of these interfacial dislocations are 60° type dislocations and the minority are Lomer dislocations. It implies that it is possible to eliminate all substrate threading dislocations with the introduction of interfacial dislocations by thermal shear stress.