Abstract Structural transformations of an epitaxial La0.8MnO3−δ thin film deposited on MgO (100) substrate by metal-organic chemical vapor deposition were conjointly studied by X-ray diffraction using a conventional 4-circles-diffractometer and by transmission electron microscopy. As reported in a previous work [Pignard et al., J. Appl. Phys., 82(9), 4445 (1997)] changes of physical properties of this material have been observed through annealing treatments: Films deposited at 650°C exhibit temperatures of semiconductor-metal and para-ferromagnetic transitions at 130 and 200 K while after successive annealings between 600 and 800°C in air, these transition temperatures increase up to 310 and 330 K, respectively. Two different film structures related to the evolution of these properties were determined at room temperature in their paramagnetic state. As a result, the film structures were found to be monoclinic and arranged in twinned microdomains. Several structural transformations resulting of annealing treatments were deduced: (i) The as-deposited film is almost single-phased with a I2/b space group structure and transforms into a mixing of two phases of I2/b and P21/a space groups in a 82/18% ratio, respectively; (ii) Mn octahedra become more regular, with significant increases of Mn–O–Mn bond angles, close to 180°. These structural changes are interpreted in a discussion related to an increase of the Mn4+/Mn3+ ratio. Besides, as a surprising feature, observations by transmission electron microscopy on La0.8MnO3–δ/MgO interfaces showed that MgO exhibits domains of Fd 3 cubic lattice and a cell parameter twice this of normal MgO. This seems to be the result of a strain occurring between the film and the MgO substrate rather than a chemical diffusion.