Maleic anhydride (MAH) was grafted to polystyrene (PS) by reactive extrusion in the presence of a free-radical initiator. The grafting reaction was confirmed by spectroscopic analyses. The amount of MAH grafted on PS was evaluated by a titration method. The polystyrenes functionalized with MAH (MPSs) were blended with polyamide 6 (PA6), and the effect of MAH units incorporated in PS on the compatibility with PA6 was examined by measuring the morphology, rheological and tensile properties. Although the content of MAH in MPS prepared in this study was much less than 1 wt%, a significant change in the morphology and rheological and tensile properties of PA6/MPS blends was observed. The formation of PS-g-PA6 copolymers during melt mixing of PA6 and MPS was evaluated using spectroscopic analyses and also confirmed from the high stability of the suspension obtained by dissolving the blend in formic acid.