The order parameter of FePt thin films plays an essential role in determining such diverse materials properties as magneto-crystalline anisotropy, magnetic coercivity and magnetic recording density. Typically, the order parameter for a bulk material is obtained by measuring the X-ray integrated intensity ratio of a super lattice peak to a fundamental peak and comparing this ratio to a theoretical value which has been calculated for a fully ordered sample. In this work, we present an analysis of the order parameter calculation in FePt L10 thin films taking into account the geometric features of the X-ray diffractometer, the crystallographic texture of FePt films, and the finite thickness of the films. The theoretical ratio of the (001) super lattice peak of FePt and the (002) fundamental peak of FePt is calculated as a function of the full width half maximum (FWHM) diffraction peaks from film and thickness for FePt thin films with perpendicular texture. A reliable order parameter calculation equation for Fe50Pt50 fiber textured perpendicular recording media is established.