Abstract: The effect of different temperature (below 1100℃) post-oxidation annealing in Ar atmosphere (Ar POA) on the densification of thermally grown SiO2 film on n-type 4H-SiC has been studied by reflective spectroscopic ellipsometry (SE) and Fourier transform infrared (FTIR) spectroscopy. The spectroscopic ellipsometry studies show that the 600℃ annealed SiO2 film has the highest refractive index of 1.47 and the lowest thickness of 84.63 nm in all samples. It is obtained from FTIR that 600℃ annealed sample has the highest LO phonon intensity, which may be attributed to the highest concentration of Si-O bonds. The leakage current-voltage measurement of Al/SiO2/SiC MOS capacitor was also performed. The leakage current is decreased by two orders of magnitude of the SiO2 thin film after annealing at 600℃. When a reverse bias voltage of 5V is applied, the reverse leakage current density is only 5×10-8 A/cm2. According to all studies, we conclude that annealing at 600℃ can greatly improve the compactness of thermally oxidized SiO2.