Aligned thin films of the liquid-crystalline polymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-bithiophene] were prepared, and the correlation between the optical anisotropy and the structural properties was shown. A series of samples with different thicknesses were prepared via a spin-casting process on rubbed polyimide surfaces. The alignment of the polymer chains was obtained by a temperature treatment just below the clearing temperature. The degree of alignment was investigated with ultraviolet-visible absorption spectroscopy and in-plane X-ray diffraction. Independently, each technique revealed Hermans orientation functions in the range of 0.75-0.8. Surprisingly, a layer-thickness dependence was not observed. In addition, the X-ray diffraction pole figure technique revealed that the polymer chains were uniaxially aligned along the rubbing direction. The aligned films were in the nematic state, with the director elongated along the rubbing direction. (c) 2007 Wiley Periodicals, Inc.