Abstract: The structure of the two step rectangle 3D braided composites with five-direction and three-demension is truly described and analyzed. Three kinds of unit cells have been divided. According to the braiding movement rule, the axial yarns keep static in the axial direction and the braiding yarns run across along the diagonal direction at an angel of 45° with the preform surface to and fro in a proper order to bind up the axial yarns. Through surveying the cross section and the section cut longitudinally at a 45° angle with the preform surface of the composites, the true cross sections and the shapes of axial and braiding yarns are described clearly. Thereby, the volume of the deformations of axial yarns being flattened by the braiding yarns and the fiber volume fraction in the axial yarn are determined.