The　performance　of　nitride　devices　is　strongly　affected　by　their　polarity.　Understanding　the　polarity　determination　and　evolution　mechanism　of　polar　wurtzite　nitrides　on　nonpolar　substrates　is　therefore　critically　important.　This　work　confirms　that　the　polarity　of　AlN　on　sapphire　prepared　by　metal–organic　chemical　vapor　deposition　is　not　inherited　from　the　nitrides/sapphire　interface　as　widely　accepted,　instead,　experiences　a　spontaneous　polarity　inversion　during　the　growth.　It　is　found　that　at　the　initial　growth　stage,　the　interface　favors　the　nitrogen-polarity,　rather　than　the　widely　accepted　metal-polarity　or　randomly　coexisting.　However,　the　polarity　subsequently　converts　into　the　metal-polar　situation,　at　first　locally　then　expanding　into　the　whole　area,　driven　by　the　anisotropy　of　surface　energies,　which　results　in　universally　existing　inherent　inverse　grain　boundaries.　Furthermore,　vertical　two-dimensional　electron　accumulation　originating　from　the　lattice　symmetry　breaking　at　the　inverse　grain　boundary　is　first　revealed.　This　work　identifies　another　cause　of　high-density　defects　in　nitride　epilayers,　besides　lattice　mismatch　induced　dislocations.　These　findings　also　offer　new　insights　into　atomic　structure　and　determination　mechanism　of　polarity　in　nitrides,　providing　clues　for　its　manipulation　toward　the　novel　hetero-polarity　devices.　©　2022　Wiley-VCH　GmbH.