We propose a modification of the interface roughness (IFR) scattering form factor in intersubband transitions. We properly derived a formula for the form factor for IFR scattering in terms of the integrals of the envelope wave functions. This new form factor has a more global natural than the old one (proposed by Ando) and may be suitable for a wide range of applications. In this paper, we calculate and compare the absorption linewidths by applying of the old form factor and the new one. Different from previous calculations, for the same surface profile (delta, Lambda), the interface roughness scattering absorption linewidth calculated with the new form factor is twice as great as that calculated with the old one. Our numerical calculations may better explain the experimental results for the well-width dependence of the intersubband absorption linewidth.