Metal-oxide-semiconductor (MOS) capacitors and MOS field-effect transistors (MOSFETs) were fabricated on C-face 4H-SiC with post-oxidation annealing in phosphorus- containing atmosphere. POCl3/N2 annealing at 1000 °C, which is an effective condition for Si-face, did not bring any improvement in the interface state density (Dit) for C-face due to additional oxide growth. We have developed a new process sequence suitable for C-face MOS structures. As a result, the Dit near the conduction band edge was drastically decreased by the developed process to less than 3x1011 cm−2eV−1. The field-effect mobility of C-face 4H-SiC MOSFETs was effectively increased to 37 cm2/Vs. We found that the incorporation of phosphorus atoms into the SiO2/SiC interface can improve MOSFET performance not only for the Si-face but also for the C-face.