This is an experimental study of the thermal properties of polypropylene composites reinforced with aligned Pyrograf III(TM) vapor-grown carbon fiber. The fiber reinforcement was uniaxially aligned in the composite samples by an extrusion process, thereby producing improved properties along the preferred direction. The density, specific heat, and thermal diffusivity were determined experimentally and used to calculate the thermal conductivity in both the longitudinal and transverse directions of the test specimens. The experimental thermal conductivity results were then compared with those predicted by the Lewis and Nielsen semitheoretical model to evaluate the axial thermal conductivity of the Pyrograf III fiber.