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In this study, wide- and small-angle scattering (WAXS, SAXS) are used to analyze the structure of polyacrylonitrile (PAN) carbon fiber. Wide angle scattering is used to identify lattice parameters such as d-spacings, stacking height of crystalline planes (Lc), average crystallize size (Lc), and various other features (void content, degree of graphitization). Small angle scattering results show orientation and size distributions of pores/amorphous regions. Several analysis techniques were applied to small angle scattering results to ensure accuracy. XRD Data collected is correlated to single fiber mechanical properties and a fiber model is developed. Fiber modulus is heavily related to crystallite dimensions and orientation as well as pore structure. Authors hypothesize that the non-linear elasticity observed in the fiber direction of carbon fibers arises from elastic planar motion within turbostratic carbon. As crystalline regions grow and expand, the lattice structure improves lowering the barriers for (002) quasi-graphene plane motion. This miniscule elastic planar motion can lead to a large nonlinear response to tensile loading especially for high crystallinity carbon fibers and underlying deformation mechanics is addressed considering transverse-isotropy of turbostatic graphite crystals.