We demonstrated the polarity control of GaN on sapphire substrate by TMA1 preflow in LP-MOVPE growth, and proposed a model of "two monolayers of Al" giving the qualitative explanation for the polarity conversion mechanisms. In the present work, we intended to extend this method to MBE growth and testify the validity of this model. Both At layers and AlN intermediate layer were used to realize the growth of Ga polar GaN by RF-MBE. Polarities of the epilayer were characterized by Coaxial Ion Collision Impact Scattering Spectroscopy. The effects of At layer thickness and AlN intermediate layer growth procedure on GaN epilayer polarity;ere investigated. The results showed that, both the AlN intermediate layer and At inserted layers could change N polarity to Ga polarity. Excess At coverage on AlN surface is crucial for polarity inversion by AlN. It is proposed that the polarity selection of GaN is a kinetic process, the polarity conversion of GaN by AIN or At relies on the fact that they provide an At platform on which the subsequent epilayer preferred to grow with Ga polarity. (C) 2002 Elsevier Science B.V. All rights reserved.