We have observed plasmid deoxyribose nucleic acid (DNA), in real space images, before and after treating with restriction enzyme (Pvu II or Hinc II) using an atomic force microscope (AFM). The enzyme Is recognized on DNA even in the absence of Mg2+ ions. In the presence of Mg2+, on the;other hand, direct evidence was obtained that the enzyme could bind to circular plasmid DNA without cutting and cleaved it at the site corresponding to the specificity. Lengths of the DNA fragments observed by AFM were consistent with the values estimated by agarose gel electrophoresis. In addition, as substrates for the AFM observation, rutile TiO2(110) single crystal surface was found to be effective in expanding and fixing the DNA molecules as straight chains along the stepped surface.