Formation of an AlInN interface transition layer in plasma-assisted molecular beam epitaxy (PA MBE) of InN/Al2O3 (0001) structure has been found by high-resolution X-ray diffraction (XRD), transmission electron microscopy (TEM), secondary ion mass-spectroscopy (SIMS) and optical transmission techniques. Having a thickness of below 100 nm, an Al content of ∼0.3 and rather sharp interfaces, the interlayer improves the quality of the main InN film, allowing its high-temperature growth without In droplet formation. XRD Θ-rocking curves width of 350 arc sec, Hall mobility of 600 cm2/V s (300 K) at electron concentration of around 1020 cm−3 have been achieved for the best InN epilayer. Employed initial growth stage affects significantly the quality of both the AlInN interface layer and the InN layer.