We have investigated the change in the in-plane anisotropy caused by a washing treatment of UV irradiated polyimide (poly[4,4'-oxydiphenylene-1,2,3,4-cyclobutanetetracarboximide]) films. The in-plane anisotropy of the films exposed to linearly polarized UV light (LPUVL) was determined by measuring the polarized infrared absorption spectra. We found that part of the polymer fragments produced by photo-induced decomposition are unstable, and that they can be removed by a few minutes of ultrasonic cleaning. In the LPUVL exposure range below 1.5 J/cm(2) , the film anisotropy slightly increases after the washing treatment and beyond this exposure it decreases. As a result, the film anisotropy becomes maximum at 1.5 J/cm(2) for the washed film, while it is maximum at similar to3 J/cm(2) for the unwashed film.