Structures of Si(001) surfaces in the initial oxidation processes at 110 K have been investigated by reflection high-energy positron diffraction (RHEPD). From the analyses of RHEPD rocking curves for various coverages of oxygen, it is found that the adsorbed oxygen atoms are located on top of the silicon dimer layer and in the dimer back-bonds. The distance between the surface oxygen and the dimer silicon, between the dimer silicon and the back-bond oxygen and between the back-bond oxygen and the bulk silicon is determined to be approximately 1.0 Å, 0.7 Å and 0.7 Å, respectively. The initial oxidation process is discussed from the change of specular spot intensity due to exposing oxygen. [DOI: 10.1380/ejssnt.2006.510]