GaAs/GaN heterostructures were grown by molecular-beam epitaxy using GaN/supphire (0001) templates. In spite of a ∼20% lattice mismatch, epitaxial growth was realized, so that the GaAs films showed good adhesion and their surface had a large mirror-like area. The GaAs films were as thick as 1um. The surface profile was characterized by atomic-force microscopy, which gave an average roughness of 10 nm for a 5×5 μm scan. Micro-Raman characterization and transmission electron microscopy (TEM) showed that the epitaxial GaAs films had zincblende lattice with (111) orientation, whereas the GaN substrates had wurtzite symmetry. The GaAs/GaN interface was found to be flat and abrupt. A large number of defects have been observed which originated from relaxation of the large lattice mismatch. The defects included misfit dislocations and nanocavities at the interface, as well as dislocations and stacking faults in the bulk of the GaAs film. Sharp interference fringes and characteristic behavior were observed for the ψ and Δ parameters of spectroscopic ellipsometry in the range of 0.75-5.3 eV. Simulation of the optical properties of the GaAs/GaN/sapphire heterostructure indicated a reasonably good optical quality of the layers and interfaces. Photoluminescence (PL) spectra recorded at the temperatures from 17 to 300 K revealed wide and weak radiative bands. Non-radiative processes dominated in recombination of non-equilibrium carriers. The observed PL broadening originated from the band tails that were a result of the high density of charged defects.