The initial reaction of the 6H-SiC(0 0 0 1)root3 x root3 surface with O-2 molecules at 500 degreesC has been in situ observed using a variable temperature scanning tunneling microscope (STM). When the surface was exposed to O-2 molecules, many bright spike noises were observed. These noises can be attributed to the O-2 molecules rapidly migrating on the surface. With an increase in O-2 exposure, dark lines (L) were formed on the surface. L was elongated toward (1 (1) over bar 00) directions, which manifests anisotropic surface etching induced by O-2 molecules. No specific site was observed for the starting point of the formation of L. The anisotropic etching process had a tendency of additional L formation at the vicinity of the former L. The process at which L formed a triangular domain was observed in real time. On this basis, the formation mechanism of the domain is discussed. (C) 2003 Elsevier Science B.V. All rights reserved.