Effects of carbon impurities and oxygen vacancies in Al2O3 film on the characteristics of Al2O3/GaN MOS capacitors were studied using the different atomic layer deposition (ALD) precursor and high-pressure water vapor annealing (HPWVA). Trimethyl aluminum (TMA: Al(CH3)3) and dimethyl aluminum hydride (DMAH: Al(CH3)2H) were used as ALD precursors to control the carbon impurities. On the other hand, oxygen vacancies in Al2O3 film were suppressed using the HPWVA. The DMAH precursor reduced the concentration of carbon impurities in the ALD Al2O3 film. It was found that the interface trap density (Dit) was mainly affected by the carbon impurities rather than the oxygen vacancies at the Al2O3/GaN interface. On the other hand, voltage stress induced flat band voltage (VFB) shift was attributed to both the carbon impurities and the oxygen vacancies.