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The fiber orientation state in carbon fiber reinforced polyamide ６ composites was expressed by an order parameter(S２)that is used to express the orientation state of Nematic Liquid Crystal. S２ was derived from the X-ray diffraction image of the composites that reflects the fiber orientation distribution. As the specimens, composites of various fiber content were prepared and, regardless of the content, S２ and their tensile properties showed positive correlation. Furthermore, we predicted the relation between the fiber content and tensile modulus in the case of the quasi-isotropic composites used by the measured tensile modulus and S２ to compare with those calculated by the modified Halpin-Tsai model that takes into account the fiber length distribution of the specimens. Because of the similarity between the results derived from the different methods, it was found that S２ is a useful index that represents the condition of fiber orientation in the fiber reinforced composite materials.