Self-organization behavior of a dinuclear metal complex, Na[Cr-2(L-tart(2)H)(phen)(2)], in water was investigated by cryo-transmission electron microscopy and atomic force microscopy(AFM). Very long ribbonlike supramolecular assemblies (300-1000 Angstrom width and 1/10 thickness) were formed in lyotropic (nematic) liquid crystal and arrayed with a certain orientation. The two-dimensional small-angle neutron scattering showed an anisotropic pattern. The characteristic Bragg peaks were observed at Q(peak) = 0.018 and 0.036 Angstrom(-1) (interlayer distance = 350 Angstrom). A Bragg peak in X-ray diffraction was observed at 2 theta(peak) = 10.55 degrees (distance = 8.38 Angstrom). The molecular order in the assemblies was confused by the nanometer observation of AFM. The molecular arrangement in the assemblies was discussed. Molecules interact through pi-pi stacking between phenanthroline rings, resulting in the formation of a sequential helical array along the assembly axis. In this arrangement, the long axis of the molecules orients along the assembly axis. In the direction normal to the assembly axis, tartrate cores arrange through hydrogen bonding.