DNA triplex modulates gene expression by forming stable conformation in physiological condition. However, it is not feasible to observe this unique molecular structure of large molecule with 54 oligodeoxynucleotides directly by conventional nuclear magnetic approach. In this study, we observed directly single molecular images of paperclip DNA triplexes formation in a buffer solution of pH 6.0 by atomic force microscopy (AFM). Meanwhile, a diffuse "tail" of unwound DNA was observed in pH 8.0 solution. This designable approach in visualizing the overall structures and shapes of oligo-DNAs at the single molecular level, by AFM, is applicable to other biopolymers as well.