Polarized Fourier transform infrared (FTIR) and ultraviolet-visible (UV-VIS) spectroscopy were used to investigate the mechanism of liquid crystal alignment on polarized UV (PUV) irradiated polyimide (PI) films. PI films strongly absorb below 320 nm resulting in photochemical reaction of the PI. PUV irradiation of the PI film caused decrease of all the peak intensities in the IR, except the newly formed 1396cm−1peak, due to degradation of the PI molecules. The preferential degradation of PI molecules parallel to PUV irradiation direction results in the predominant orientation of the remaining PI molecules perpendicular to the PUV irradiation direction. But the rubbing of the PI films induced reorientation of the PI molecules parallel to the rubbing direction.