The trimethylaluminum (TMAl) preflow process has been widely applied on sapphire substrates prior to growing Al-polar AlN films by metalorganic chemical vapor deposition. However, it has been unclear how the TMAl preflow process really works. In this letter, we reported on carbon's significance in the polarity and growth mode of AlN films due to the TMAl preflow. Without the preflow, no trace of carbon was found at the AlN/sapphire interface and the films possessed mixed Al- and N-polarity. With the 5 s preflow, carbon started to precipitate due to the decomposition of TMAl, forming scattered carbon-rich clusters which were graphitic carbon. It was discovered that the carbon attracted surrounding oxygen impurity atoms and consequently suppressed the formation of AlxOyNz and thus N-polarity. With the 40 s preflow, the significant presence of carbon clusters at the AlN/sapphire interface was observed. While still attracting oxygen and preventing the N-polarity, the carbon clusters served as randomly distributed masks to further induce a 3D growth mode for the AlN growth. The corresponding epitaxial growth mode change is discussed.