The microstructure of m -plane InGaN epilayers grown on m -plane ZnO has been found to depend significantly on indium composition for the range from 0.07 to 0.17, where anisotropic lattice mismatch between InGaN and ZnO results in decreasingly tensile and increasingly compressive stress along the a and c lattice axes, respectively. For indium compositions below ∼0.10, periodic arrays of misfit dislocations with a Burgers vector of 1/3 [11 2- 0] are observed parallel to the [0001] direction. For indium compositions above ∼0.12, generation of basal-plane stacking faults relieve the compressive stress along the [0001] direction. These characteristic mechanisms of strain relaxation should provide new approaches to engineer thick InGaN layers with reduced lattice misfit strain. © 2010 American Institute of Physics.