A series of rigid-rod polyesters with long alkyl side chains, denoted as B-Cn, have been prepared by condensing 1,4-dialkyl esters of pyromellitic acid with biphenol. The alkyl side-chain lengths were varied from hexyl (n = 6) to octadecyl groups (n = 18), and the thermotropic transition behavior and mesophase structure of these materials were examined by optical microscopy, DSC, and X-ray measurements. An increase in the length of the side chain decreases remarkably the melting point of the crystalline phase, giving rise to a liquid crystalline phase. Two types of liquid crystalline phases with layerlike modifications, LC-1 and LC-2, have been observed. The transition behavior of these liquid crystalline phases is described as a function of the side-chain length. In addition, the structural characteristics of the liquid crystalline phases as well as the crystalline phase are described as based upon X-ray data collected for oriented specimens.