The effect of polystyrene modified with maleic anhydride (MPS) on the compatibility of the immiscible blends of polyamide 6 and polystyrene was investigated as a function of the molecular weight of MPS. The MPS was prepared by melt extrusion in a twin screw extruder and the content of maleic anhydride in the MPS was below 1 wt%. The compatibilizing ability of the MPS was examined through several experimental techniques such as scanning electron microscopy and measurements of mechanical and rheological properties. The interfacial adhesion between two separated phases was also measured using a butt joint test. The compatibilizing ability of the MPS was very dependent on the molecular weight of the MPS. The MPS of high molecular weight seems to be more effective in reducing the domain size of dispersed phase and in increasing the interfacial adhesion.