Depth profiles of oxygen, carbon and silicon atoms in oxide layers on the Si face of 4H-SiC (0001) epitaxial film were measured by using Secondary-Ion Mass Spectroscopy (SIMS). We find that the transitional region exists at the SiO2/4H-SiC interface. There exists compositional fluctuation in the transitional region. Conditions of the oxidation and the post-oxidation anneal processes affects thickness and composition of the transitional region. Capacitance-Voltage (CV) analyses were also performed for MOS-diodes. Thickness of the transitional region derived from SIMS analyses relates to the amount of interface traps obtained from the CV analyses.