The AlN film with GaN initiallayer (GaN buffer layer and epilayer)has been grown on αAl2O3 (0001)substrate by electron cyclotron resonanceplasmaenhanced metal organic chemical vapor deposition(ECRPEMOCVD) technique at low temperatures using TMAl and high pure N2 as Al and N sources, respectively. The effects of hydrogen plasma cleaning, nitridation and GaN initallayer on the quality of AlN epilayer have been investigated by RHEED(reflection highenergy electron diffraction) , TEM(transmission electron microscope) and XRD(xray diffraction). And highquality hexagonalphase AlN single crystal films whose cleavability is the same as the substrate have been grown at low temperatures. The full width at half maximum of XRD peaks is 12′.