Adsorbate effect of the surface structure of 6H-SiC(0001)root3xroot3- R30degrees surfaces has been studied by rocking curves of reflection high energy electron diffraction (RHEED) intensities and Auger electron spectroscopy (AES). Two types of root3xroot3 surfaces, silicon rich and carbon rich surfaces, were observed in a series of an annealing process. In order to find what causes the phase transition from the silicon rich to the carbon rich phases, following gases are exposed on the silicon rich root3xroot3 surface; H, O-2 and a mixture gas of H-2 and O-2. In the case of hydrogen-treated root3xroot3 surface, the RHEED rocking curves and the Auger spectrum show good agreement with those of the carbon rich surface. It is concluded that the rearrangement of the root3xroot3 surface in UHV is caused by adsorption of hydrogen on the silicon rich root3xroot3 surface.