The magnetic domain structures of ultrathin Co films, epitaxially grown on Ni(110) substrate, were studied by a Kerr microscope combined with a digital image processing system for the first time. The digital image processing system allows us to subtract contrast of nonmagnetic origin from the magneto-optic image. It was found that the Co(110)/Ni(110) system exhibits domain structures different from that of the Ni(110) substrate. Irregular domains of ultrathin Co films larger than 2 mm were observed, while plate(stripe)-like domains were found for the Ni(110) substrate. Furthermore, the domain patterns of the Co(110)/Ni(110) system are strongly influenced by the adsorption of gas and variation of temperature.