The process of thermal oxidation of a H-terminated surface of 4H-SiC(0001) Si-face was investigated using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and in-situ Fourier-transformed infrared reflection absorption spectroscopy (FTIR-RAS). H-terminated SiC surfaces were obtained by H-2 annealing. Surface Si-H bonds were monitored by observing the Si-H stretching vibrations. It is found that the surface Si-H bonds are stable against annealing up to 400degreesC in UHV while no change in Si-H stretching vibration spectrum is observed. In contrast, the Si-H stretching vibration shifted toward a higher frequency by annealing at 400degreesC in 02 ambient. It is considered that the frequency shift of Si-H stretching vibration is derived from the formation of OxC3-xSi-H species by oxidation. This result indicates that the early stage of thermal oxidation on 4H-SiC surface could be revealed by observing change for the Si-H stretching vibration.