Using a convenient sonochemical method, we have prepared polystyrene (PS) functionalized graphenes (FGs) by direct exfoliation of graphite flakes in the monomer of styrene. This material could be dispersed in toluene and subsequently formed into composites with PS by solution casting followed by a compression molding method. The FGs are well dispersed in the PS matrix as evidenced by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The addition of a small amount of FGs significantly increased the glass transition temperature (T-g) of the PS matrix. It's worth noting that compared to pure PS, a 103.7% increase in tensile strength and a 407% improvement of Young's modulus were achieved by addition of only 0.5 wt% FGs due to the effective load transfer between the FGs and the PS matrix. The storage modulus was also considerably enhanced upon addition of FGs. The PS/FG composites show superior mechanical properties as compared to their counterparts reinforced by carbon fillers such as multi-walled carbon nanotubes (MWNT) and thermally reduced graphene (TrG). The presence of PS on the FG surfaces and thus a strong interaction between the FGs and PS is essential for the property enhancements. Moreover, the Young's modulus of the PS/FG composites was compared with the predictions of the well-established Halpin-Tsai model. The FGs are very effective as a reinforcing filler for graphene-based polymeric composites.