Anisotropy in a polyimide film was investigated in a quantitative manner by a nonlinear regression of the thermal shrinkage data obtained from thermomechanical analysis. The thermally induced shrinkage of this pyromellitic dianhydride-oxydianiline polyimide film at 573 K was directionally anisotropic in time film plane by as much as about 0.4%. The direction of maximum thermal shrinkage was inclined by about pi /6 rad from the machine direction of the film. The thermal shrinkage behavior of the polyimide films in the vicinity of the glass-transition temperature (T-g) showed an unusual anisotropic response. On the basis of a correlation between the anisotropy in the thermal shrinkage of the films and the molecular orientation of the polyimide, this characteristic thermal shrinkage behavior around T-g is suggested to be due to a recovery of the free volume lost by a physical aging process.