The influence of RF power on the growth of N-polar AlN thin layers on Si(111) by plasma-assisted molecular beam epitaxy was systematically studied. The surface reconstruction of AlN was 1 × 1 for samples grown at RF power of 350-450W, while samples grown at lower power of 150-300W had 3 × 6 reconstruction. All samples demonstrate smooth surface with root mean square roughness less than 1 nm. The 45nm thick AlN sample grown at RF power of 150W has the best structural quality. The screw and edge dislocation densities estimated for this sample were 1.4 × 109 and 5.9 × 109cm-2, respectively.