5, 10, 15, 20-tetrakis-(4-chlorophenyl) porphyrin, H2TTPCl4, has been isolated as a new polymorph following the modified Adler's method and its X-ray crystal structure solved. The new polymorph (I) crystallises in the monoclinic space group, P2(1)/n with a = 10.1574(5) angstrom, b = 8.9827(4) angstrom, c = 20.9350(8) angstrom, beta = 102.532(4)degrees, V = 1864.62(15) angstrom(3), Z = 2. The previously found polymorph, prepared using Lindsay method was crystallised in monoclinic space group, P2(1)/a with a = 15.776 (13) angstrom , b = 8.646 (3) angstrom, c = 14.087 (5) angstrom , beta = 96.05 (5)degrees, V = 1910.7 (3) angstrom(3), Z = 2. The main difference between the two polymorphs seems to be the different packing arrangement of molecules in their crystal lattices. The dramatic self aggregation property of new polymorph (I) has also been investigated. The investigation reveals that under certain conditions of solute concentration and pH of the media, the compound exhibits a strong tendency to exist in a prominent self aggregated state in head-to-tail type (j-aggregation) molecular alignment. The self association behaviour of (I) was confirmed by UV-Vis absorption spectra, performed in chloroform by varying concentrations and pH changes and H-1 NMR spectra, performed in deuterated chloroform at varying concentrations and their results have been discussed in detail.