We have investigated post-oxidation-annealing (POA) effects on thermally grown SiO2/n-type 4H-SiC interface. The flat-band voltage shift DeltaV(FB) and the interface state density D-it, could be improved by POA both in Ar and H-2. High temperature POA in H-2 above 800 degreesC drastically reduced the D-it it is thought to be a result of the termination of the dangling bonds at the SiO2/SiC interface by hydrogen atoms.