Thermally grown silicon oxide films on p-type 6H-silicon carbide substrate under different oxidation and nitridation conditions have been characterized by x-ray reflectivity technique. Electron density profile obtained from the analysis of the x-ray reflectivity data shows that the thickness, density of the oxide film and structure of the oxide-SiC interface strongly depend on the different growth conditions. In particular, the density of the oxide film for all the samples other than that grown in NO is found to be much lower and also not fixed within. It is maximum at the interface and gradually decreases towards the top in all samples except for the sample grown in O2 followed by NO nitridation. For the latter, a very low density at the interface region has been observed. The sample grown in NO shows the best performance in capacitance-voltage characteristic and reliability studies suggesting that the bad performance of the oxide grown on p-type SiC system as metal-oxide-semiconductor devices is mainly linked to the low density oxide film and can be overcome under proper growth condition.