Molecular orbital (MO) calculation and molecular dynamics (MID) simulation were carried out for a set of smectic liquid crystalline molecules, 8O-O8 (4-octyloxyphenyl-4 ' -octyloxybiphenyl-4-carboxylate) and 8O-8 (4-octylphenyl-4 ' -octyloxybiphenyl-4-carboxylate), to understand the molecular origin of different crystalline structure formation and to predict their conformational property in liquid crystalline phases, The results of MO and MD analyses indicate that the structure of 8O-O8 and 8O-8 in crystalline phase is essentially determined by intramolecular interactions and the experimentally observed structure is chosen as a consequence of intermolecular packing constraints. This paper also reports the first application of two-molecular ab-initio calculation to liquid crystalline systems.