Al thin films were grown on MgAl2O4 spinel substrates using solid-source molecular-beam epitaxy. The structural properties of Al layers were systematically investigated as a function of substrate orientation and growth temperature. Scanning electron microscopy and atomic force microscopy show that low growth temperatures lead to smoother and more coalesced Al films. X-ray diffraction and electron backscatter diffraction (EBSD) measurements demonstrate that Al layers are single crystalline and coherently grown on both (100)- and (111)-oriented spinel substrates. EBSD data also clearly reveal a high density of twin domain structures in Al films grown on (111) spinel substrates, and all twin boundaries were determined to be Σ3 boundaries. High-resolution transmission electron microscopy was used to confirm the presence of twin structures. Al grown on (001) spinel roughens more easily than Al grown on (111) spinel at higher growth temperatures. It is suggested that Al surface energy and thermal expansion mismatch play a critical role in the evolution of surface morphology of Al thin films grown on MgAl2O4 spinel.