A theoretical framework for calculating XPD intensity that incorporates relativistic effect is developed. The theory is applicable to the cases where relativistic effects become important, for example, such a case as heavy elements (Z > 50) are present or circularly polarized photons are used in initial excitation of photoelectrons. The theory is based on short-range-order-multiple-scattering approach so that it is practically useful in analyzing experimental data in order to obtain structural information. In this framework relativistic Dirac Green's function is expanded in terms of full non-relativistic Green's functions using Gestzesy expansion. Using this formalism explicit formulae for spin-resolved XPD from s (l = 0) core levels are given by truncating the expansion at first few terms, which is sufficient in most cases. Numerical examples for direct (atomic) photoemission and single scattering XPD in two-atom models are presented. The theory has an advantage that we can use well-defined Debye-Waller factors and optical potentials developed within the framework of non-relativistic theory because the expansion is possible in terms of non-relativistic Green's functions.