The addition of end-functionalized polyamide 66 (fPA) greatly improved the compatibility of fPA/polyether imide (PEI) blends. The compatibility and interaction of the components in the blends were characterized by Fourier transform-infrared spectra, Molau tests, melt flow index, dynamic mechanical analyses, heat deflection test, Izod impact test, and scanning electron microscopic observations. The results show that the epoxy end group in fPA can react with the amino or the carboxylic end groups in PEI. In this way, the in situ-formed fPA-b-PEI copolymer anchors at the interface of fPA/PEI blends. The morphological study of the fracture sections before and after impact test reveals that the PEI particles were dispersed uniformly in the fPA matrix and the interfacial adhesion between fPA and PEI was increased significantly compared to the unmodified PA66/PEI blend. The thermal mechanical properties of the blends thus were enhanced with increasing degree of functionalization of fPA. Next the fPA content, with the highest degree of functionalization at 3.4 %, was varied from 0 to 60 wt%; whereas, the content of PEI was held at 40 wt% in the PA66/fPA/PEI blends, and in all cases the PEI formed the dispersed phase. From various characterization results, the peak property improvement was reached at 20 wt% fPA. Besides, most high-temperature properties of the ternary blend were raised over the neat PA66.