The iodine-modified Ag(111) substrate was employed as an electrode to investigate the adsorption of 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)-21H,23H-porphine tetrakis(p-toluenesulfonate) (TMPyP) in an alkaline solution containing KI. It was found by using in situ scanning tunneling microscopy (STM) that water soluble TMPyP molecules were irreversibly adsorbed and formed highly ordered molecular adlayers on the surface of the iodine-modified Ag(111) electrode within the potential range of -0.35 to -0.10 V vs Ag/AgI. STM images revealed the characteristic shape, internal molecular structure, and molecular orientation of each TMPyP molecule in ordered adlayers. The ordered TMPyP adlayer usually consisted of stripes of several straight molecular rows with different orientations along the root 3 direction of the underlying iodine adlayers. In each row, molecules were aligned with the same orientation. From the time-dependent imaging, the ordered molecular adlayer was found to grow in the specific root 3 direction with an averaged growth rate of ca. 6 nm/s.