The surface morphology and the crystal structure of very thin intermediate Mg-Ti-O films grown on MgO(100) were studied by use of non-contact atomic force microscopy, X-ray diffraction and low-energy electron diffraction. When Ti was deposited in an O-2 atmosphere on a MgO(100) substrate maintained at 600 degreesC, rectangular structures of Mg-Ti-O spinel nano-crystals were formed. When the surface was heated at 900 degreesC in an O-2, atmosphere, a long-range stripe structure was observed. The structure is the modulated structure of a M-Ti-O spinel surface. After prolonged heating, a titanium oxide-MgO interfacial reaction proceeded, the stripe structure disappeared and atomically flat terraces were observed. Several different morphologies of the Mg-Ti-O spinel films were obtained with different deposition conditions.