The polarity-controlled growth of GaN on a sapphire substrate by metalorganic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) was demonstrated. The mechanisms for polarity reversion of GaN by TMAl preflow in MOVPE growth and high-temperature deposited AlN intermediate layers in MBE growth were discussed based on the 'two monolayers of Al' model. The kinetic process of GaN polarity selection on a sapphire substrate, Al layers, and AlN urface was investigated by RF-MBE growth. Reversing Ga polarity to N polarity could also be realized by nitridation of the deposited Al layers. These results provided a comprehensive understanding of the effects of surface stoichiometry, growth temperature and N source species (ammonia or N plasma) on GaN polarity. It was concluded that GaN tended to grow with Ga polarity which was kinetically favorable on thermally cleaned sapphire substrates and Al-covered surfaces, the polarity conversion of GaN by TMAI preflow, AlN intermediate layer or Al insertion layers relied on the fact that they provided an Al platform on which the subsequent epilayer prefers to grow with Ga polarity.