The feasibility of mixing hardwood aspen pulp [chemithermomechanical pulp (CTMP)] and surface-treated mica as the hybrid reinforcing filler for polystyrene composites has been investigated by evaluating the mechanical properties of the resulting composite materials. Compatibility of wood fiber and polystyrene, as well as mica, was improved by surface treatments of CTMP, for example, coating with polystyrene + isocyanate or with silane and grafting with polymer. Mechanical properties deteriorated when nontreated CTMP was used in addition to treated mica. However, properties improved when surface-treated CTMP was used as a hybrid filler. Experimental results indicate good compatibility between treated CTMP/mica and polystyrene composites.