A novel analytical method for predicting the inclination of a resist mask during its trimming process has been proposed by considering the formation of the surface degraded layer with high compressive stress. It was found that the irradiation of oxygen radicals creates a degraded layer on the resist surface and causes high compressive stress in it. A nonuniform spatial distribution of oxygen radicals, therefore, causes an asymmetrical stress field on the resist-mask surface. Such an asymmetrical stress field distorts the resist mask. The well-known Deal-Grove's oxidation model was modified for the trimming process by considering the time-dependent change of the spatial distribution of oxygen radicals on the mask surface. This model successfully explains the observed complicated time-dependent deformation of the resist mask.