With increasing coverage,the adsorption of oxygen on W(110) surface can be- subdivided into chemisorption,two-dimensional oxidation and bulk oxidation. In this work,the new imaging technique low energy electron microscopy (LEEM) was used to investigate the two-dimensional oxidation on W(110) surface. With increasing O coverage,the low energy electron diffraction patterns change from (1×1) to p(2×1) structure,and then into p(2×2) with complex diffraction satellites. Using dark field LEEM mode,we have imaged the fractional spots near (00) beam. It was found that there were two kinds of regions with opposite contrast on the surface,i.e. the oxygen superstructure domains with different orientations. The distribution of these domains was related to surface defects,especially to surface steps. It was also indicated that the temperature played a very important role for surface oxidation.