The thickness of transition layer of a crystalline polymer is generally calculated by one-dimensional electron density correlation function. If the transition layer of the crystalline polymer is diffusive, the corrected Porod's law is an effective method to deal with the diffuse transition layer of pseudo two-phase material. The method of Porod's law is used to compute the thickness of transition layer of crystalline polymer and compared with that of correlation function. The conclusion is that the two methods obtain identical thickness of the transition layer.