A maleic anhydride (MAH) was grafted to LDPE by the reactive extrusion in a single-screw Brabender extruder, equipped with a prototype static or dynamic mixer in the presence of a free-radical initiator-benzoyl peroxide (BP). It was found that the highest efficiency was for the dynamic mixer, next the static mixer and the lower for the shaping extruder＇s head. Here, the grafting process is not accompanied by cross-linking side reactions of the polyethylene chains. For LDPE and PA6 blended in the presence of 5 wt.% of LDPE-g-MAH by using the extruder equipped with a typical shaping head, clearly improvement of compatibility of the mixed components was not observed and poor interfacial adhesion was noticed. This evidences that a single-screw extruder equipped with a typical extrusion-shaping head is a low-efficient mixer for the mechanochemical compatibilization of the immiscible polymers. The use of the dynamic mixer essentially improves the compatibility of the mixed components. Obtained in situ a graft copolymer LDPE-g-PAG is created both because of reaction of the end amine groups in PA6 with carbonyl groups in MAH and because of recombination of the macroradicals generated under action of shearing stresses. Good stability of the phase structure during reprocessing and heating and good mechanical properties of the blend suggest that the dynamic mixer be suitable for reactive processing.