A tetrafunctional epoxy resin was modified with polyetherimide (PEI) to improve its fracture toughness. To improve the high-performance epoxy toughening, the effective functional groups were introduced for pristine PEI. PEI containing a ethylamine group as a functionality was obtained by reacting it with ethylenediamine. In the mode I fracture test, the K-Ic values of aminated PEE-toughened epoxies were higher than that of pristine PEI-toughened epoxy. The maximum K-Ic value was obtained when the interface between the PEI domain and the epoxy matrix was strengthened without much disturbing of the spherical domain shape. In addition, it was also observed that the toughening effect of aminated PEI depended an the amount of curing agent. The flexural strengths of aminated PEI-toughened epoxies were also higher than that of pristine PEI-toughened epoxy. However, when excessive amination was performed, the flexural strength of aminated PEE-toughened epoxy decreased due to the reduction of physical properties of PEI.