Multi-wall carbon nanotubes filled with iron nanoparticles were combined with polystyrene to evaluate interface interactions and nanotube orientation in composite using magnetic susceptibility measurements. Iron-containing species were introduced into MWCNT cavities as the result of catalytic chemical vapor deposition synthesis using ferrocene as a catalyst source. Polystyrene loaded with certain quantity of MWCNTs was uniaxially stretched to provide the nanotube alignment. Magnetic susceptibility measurements performed in three perpendicular directions of magnetic field confirmed the alignment in the stretching direction. The composites showed a large diamagnetic response in a magnetic field perpendicular to the nanotube axis and low response in a parallel field. In a quantitative sense, anisotropy exceeds by more than an order of magnitude the effect expected from intrinsic susceptibility of nanotubes. Apparently, the graphitic nature of the nanotube lattice results in strong non-covalent interactions with uniaxially stretched polymer matrix, and aromatic rings as side groups of polystyrene align parallel to the nanotube surface, contributing to strong diamagnetism. As magnetic susceptibility is a penetrative but non-destructive type of measurement, it can successfully characterize both the alignment of one-dimensional or two-dimensional carbon allotropies and the arrangement of the macromolecules around them, contributing to the optimal design and performance of nanocomposites.