We find that threading dislocations can play a key role in misfit strain relief in InGaN/GaN heterojunctions. Threading edge dislocations with Burgers vector b=1/3 〈 112̄0 〉 change propagation directions in the strained InGaN layer. The inclination of edge dislocations is in a direction that creates vacancy planes, which accommodate part of the misfit strain. On the other hand, mixed-type dislocations with b=1/3 〈 112̄3 〉 relieve part of the misfit strain by opening up as surface pits. The difference in the relaxation mechanism is attributed to the much different magnitude of the Burgers vector. For b=1/3 〈 112̄3 〉, the dislocation tends to be coreless, especially when under strained conditions. © 2005 American Institute of Physics.