Summary: The effect of peroxide functionalization of carbon nanofibers (CNF) on the physical and mechanical properties of polystyrene (PS)–CNF nanocomposites prepared via melt mixing was studied. The CNF functionalization was evidenced by Raman spectroscopy, comparing the ratio of peaks at 1 371 and at 1 590 cm−1 (peaks related to the disordered sp3‐hybridized carbon atom and to the graphitic structure of the sp2‐hybridized carbon atoms, respectively). The variation of the storage (E′) and tensile modulus (E) of the PS–CNF composites as a function of the untreated and peroxide treated CNF concentration were evaluated. Three different peroxide concentrations were used for treating the CNF. It was found that both E′ and E increase with CNF concentration and, in addition, increase further with the peroxide treated CNFs. Nonetheless, it was found that the greater the peroxide concentration used in treating the CNF, the greater the PS degradation via free radical attack on the polymer chain, with the corresponding negative effect on the storage and tensile modulus. Dispersion of the CNF was assessed using scanning and optical microscopy, and the positive effect of the peroxide treatment on the dispersion of the CNF is evidenced.Tensile stress‐strain behavior of PS/CNF nanocomposites.magnified imageTensile stress‐strain behavior of PS/CNF nanocomposites.