Nitrided SiO2 has been thermally grown on n-type 4H silicon carbide substrate. The effects of post deposition annealing temperature (650 to 950 degrees C) in vacuum on physical properties of the oxides have been reported. Based on Fourier transform infra-red analysis, Si-O-Si bonding of the oxide has been weakened as the annealing temperature has been increased. The increment in annealing temperature has caused the increment in oxide thickness. Refractive index has been measured via Filmetric system and the index has been gradually reduced as a function of annealing temperature. This reduction trend is also being observed in the calculated values of film density and dielectric constant. In contrast, an increment trend has been recorded in film porosity as the annealing temperature increased. These observations may be attributed to the weakening of Si-O-Si bonding in the bulk oxide and roughening of the oxide surface, which has been demonstrated by atomic force microscopy. (C) 2008 Elsevier B.V. All rights reserved.