InGaN quantum wells, grown on non-polar m-plane GaN and emitting light at 560 nm, experience lattice mismatch strain relaxation by the generation of stacking faults. Each stacking fault terminates a basal plane from the substrate side, generating misfit dislocations that have a Burgers vector with a 1/2[0001] component. The structural and optical properties of such thin film structures are reported. © 2009 The Japan Society of Applied Physics.