The interfacial structural properties of compact InN films and of noncoalesced three-dimensional InN islands, grown by molecular-beam epitaxy on Ga-face GaN/Al2O3 (0001) substrates, were investigated by transmission electron microscopy. Compact film growth was accomplished employing an InN nucleation layer, grown at low substrate temperatures. A 60° misfit dislocation network effectively accommodated the lattice mismatch in the InN/GaN interface in both cases of epilayers. The lattice constants of InN were determined by electron diffraction analysis, revealing a 0.28% larger in-plane parameter of the compact InN film relative to the corresponding lattice parameter of the InN islands. This is attributed to thermal tensile strain developed during post-growth cooling down of the epilayers, which also compensated the remaining compressive strain originating from the in-plane lattice mismatch of InN and GaN.