We report a direct observation adopting `face-on' and `edge-on' stacking styles in monolayer of a novel porphyrin derivative on highly ordered pyrolytic graphite (HOPG) using scanning tunneling microscope (STM). The porphyrin derivative, 5-hydroxyphenyl-10, 15, 20-trikis(4-dodecyloxyphenyl)porphyrinatozinc(II) (ZnDPPOH) is synthesized and assembled on HOPG. In the STM images, we observed two kinds of ordered domains. One consists of ZnDPPOH dimmers, adopting `face-on' stacking style, the other exhibits a segmented columnar structure, like piled-up `armchairs', lying down on the substrate, adopting an `edge-on' stacking style. In the latter case, the conjugated and highly delocalized pi-systems play key roles in stabilizing the monolayer. (C) 2005 Published by Elsevier B.V.