Molecular surface structures of the 4,4＇,7,7＇-tetrachlorothioindigo crystal, which possessed the excellent photoconduction property, were studied with atomic force microscopy. On the (001) surface, the two-dimensional lattice was slightly enlarged compared with the bulk data, and two kinds of translationally inequivalent molecules could be distinguished in the image. By comparison of the lattice of the monolayer, it was indicated that the molecular planes on the surface were more parallel than those expected from the bull;. structure and less parallel than those of the monolayer, and as a result, the HOMO and LUMO that are delocalized in the indigoid part were more likely to be exposed on the surface. On the (110) surface, the two-dimensional lattice expected from the bulk structure almost remained intact on the surface. It was also confirmed on the (110) surface that the molecular planes in two kinds of the stacking columns were nearly parallel to the (110) and that the molecules in one of the stacks were further rearranged in such a way that their molecular longer axes were inclined to the (110) surface. Both the relaxation of the lattice on the (001) surface and the molecular rearrangements on the (110) surface suggested that the intermolecular interactions with stacking neighbors along the c-axis strongly predominated over the surface of the tetrachlorothioindigo crystal.