The adlayer structure of 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)-21H,23H-porhine tetrakis(p-toluenesulfonate) (TMPyP) at an iodine-modified Pt(100)(I-Pt(100)) surface was investigated in HClO4 solution by in situ scanning tunneling microscopy (STM). The structure of the iodine adlayer was (root 2 x 5 root 2)R45 degrees. TMPyP molecules were adsorbed with flat-lying orientation at the I-Pt(100)) surface and aligned along the [100] direction that was parallel to the row of iodine atoms. The van der Waals type interaction between TMPyP and the iodine adlayer seems to be relatively weak because of the characteristic structure of iodine, which allows TMPyP to diffuse on the iodine adlayer on Pt(100) in specific directions to form a highly ordered array. The observed structure of TMPyP was very different from that found previously on I-Au(111), I-Ag(111), and I-Pt(111). A square lattice was found to form on I-Pt(100) with a side-by-side configuration. It is concluded that the structure of the iodine adlayer plays an important role in the formation of the ordered structure.