In this paper, Mg0.12Zn0.88O/ZnO heterostructures were fabricated on e-plane sapphire (Al2O3) substrates by plasma-assisted molecular beam epitaxy (P-MBE). The quality of the Mg0.12Zn0.88O alloy thin film was characterized by X-ray diffraction (XRD) and reflection high-energy electron diffraction (RHEED). Optical properties of the Mg0.12Zn0.88O/ ZnO heterostructure were studied by absorption and photoluminescence (PL) spectra. At room temperature (RT), Mg0.12Zn0.88O/ZnO heterostructures show two absorption edges originating from ZnO and Mg0.12Zn0.88O layers, respectively. In PL spectra, two ultraviolet emission bands related to the ZnO layer and the Mg0.12Zn0.88O layer were observed. The emission band from Mg0.12Zn0.88O layer dominates at moderately lower temperature, and the luminescence of ZnO becomes gradually important with increasing temperature. This is suggested to exist as a potential barrier in the interface and to restrict the relaxation of the carriers from the Mg0.12Zn0.88O layer to ZnO layer. As the thickness of ZnO layer decreases, the emission from the Mg0.12Zn0.88O layer becomes weaker and weaker. When the ZnO thickness is up to 2 nm. only the luminescence of the ZnO layer is observed at RT. The quenching of the emission corresponding to the MgZnO layer indicates the existence of a strong injection process in the samples with thinner ZnO layer. © 2005 Elsevier B.V. All rights reserved.