Rigid-rod aromatic B-Cn polyesters with long alkyl side chains, composed of 1,4-dialkyl esters of pyromellitic acid and 4,4’-biphenol, form two distinct types of crystalline phases, Kl and K2, depending on the length of alkyl side chain. These crystalline phases are characterized by a layered structure, in which the aromatic main chains are packed into a layer with the side chains crystallized in the space between the layers. The detailed structures for these two crystalline phases were studied by C-13 solid-state NMR spectroscopy from which it was found that there are remarkable differences in the main-chain conformation and in the packing structure of the side chains. In the K1 crystal formed by the B-Cn polymers with shorter side chains, the aromatic main chain assumes a twisted conformation with the side chains packed into an orthorhombic lattice. In contrast, in the K2 crystal of the longer side-chain B-Cn polymers, the main chains assume a coplanar conformation with the side chains packed into a triclinic lattice. The NMR studies have also been extended to the two types of layered mesophases, LC-1 and LC-2, formed by these polymers. In these phases, the side chains are molten, with the main chains in the LC-1 and LC-2 phases assuming conformations similar to those in the K1 and K2 crystals, respectively. FPT-INDO calculations for the conformational energy have shown that the coplanar conformation in the K2 and LC-2 phases is strongly disfavored whereas the twisted conformation is stable. It is believed that the disfavored coplanar conformation observed in the K2 and LC-2 phases is forced by the layered segregation of the aromatic main chains and the long aliphatic side chains.