Growth and polarity control of GaN and AlN on carbon-face SiC (C-SiC) by metalorganic vapor phase epitaxy (MOVPE) are reported. The polarities of GaN and AlN layers were found to be strongly dependent on the pre-growth treatment of C-SiC substrates. A pre-flow of trimethyaluminum (TMAl) or a very low NH3/TMAl ratio resulted in Al(Ga)-polarity layers on C-SiC. Otherwise, N-polarity layers resulted. The polarities of AlN and GaN layers were conveniently determined by their etching rate in KOH or H3PO4, namely the etching rate on N-polarity is substantial larger, a method reported earlier. We suggest that the Al adatoms form several Al adlayers on C-SiC and change the incorporation sequence of Ga(Al) and N leading to a metal polarity surface. In addition, the hexagonal pyramids, typical on N-polarity GaN surface, are absent on N-polarity GaN grown on off-axis C-SiC owing to high density of terraces on the substrate surface. The properties of GaN layers grown on C-SiC have been studied by X-ray diffraction and are reported in this paper.