X-ray studies were performed for the oriented smectic mesophases of BB-5 and BB-6 polyesters, which were prepared from p, p-bibenzoic acid and penta-and hexamethylene glycols, respectively. Well-oriented X-ray patterns were observed, offering sharp inner and broad outer reflections. From the diffraction geometry of both reflections, it could be concluded that different types of smectic mesophase are formed between BB-6 and BB-5. In BB-6, an average axis of the mesogenic group as well as the polymer chain axis lies perpendicular to the smectic layer planes. In BB-5, in contrast, the mesogenic groups are oriented with a tilt angle of about 25 to the layer normal within a layer, while the polymer chain axis lies perpendicular to the layer planes. The packing of mesogenic groups andpolymer chains into a smectic layer structure was examined by selecting the conformations of polymer so as to conform to the orientational order of the mesophase. The selected conformations appear with a different arrangement of mesogenic groups to the polymer chain axis between BB-5 and BB-6. As a result, the smectic layer structures satisfying the X-ray observations could be described such that in BB-6 the normal smectic A mesophase is formed, while in BB-5 a new type of smectic C structure is obtained in which the tilt direction of mesogenic groups is invariable in every second neighboring layer but opposite to each other between neighboring layers.