A series of composites were prepared by extruding a mechanical mixture of 90 wt-% atactic polystyrene and 10 wt-% graphite fiber. The orientation degree of both components were determined as functions of processing temperature and demension of extruding orifice by two different methods: The degree of orientation of distributing graphite fiber was evaluated by using an X-ray analyzer; while that of matrix polystyrene was determined by a thermal retraction at a temperature above the glass transition temperature. The orientation of filler, obtained from the intensity distribution curve for the azimuthal scanning of 020 reflection, was at least 80% for all samples, whereas matrix polystyrene was almost always randomly oriented in all composite samples. This may be attributed to the difference in the relaxation times for both components, because the filler fiber is much larger and stiffer than the polystyrene molecules at processing temperature. The dynamic mechanical properties of these composites were measured at temperatures from 20 to 120 C. The effect of graphite fiber was evently observed in both storage and loss moduli: The storage modulus was fairly improved and the loss component was also increased in proportion to the increase of the storage modulus by mixing graphite fiber. The former is simply understood as a reinforcing effect of graphite fiber. The latter, however, is difficult to be explained, but possibly may be interpreted in terms of a weak interaction at the interfacial boundaries between both component materials.