Interfacial properties and microfailure modes of electrodeposition (ED)-treated carbon fiber-reinforced polyetherimide (PEI) toughened epoxy composite were investigated using microdroplet test and the measurement of surface wettability. ED was performed to improve the interfacial shear strength (IFSS). As PEI content increased, IFSS increased due to enhanced toughness and plastic deformation of PEI. In the untreated case, IFSS increased with adding PEI content, and the IFSS of the pure PEI matrix showed the highest. On the other hand, for the ED-treated case IFSS increased with PEI content with rather low improvement rate. In the untreated case, neat epoxy resin appeared brittle microfailure mode, whereas the pure PEI matrix exhibited a more likely ductile microfailure mode. In the ED-treated case, neat epoxy exhibited a more ductile fracture than that of the untreated case. Critical surface tension and polar surface free energy of ED-treated carbon fiber was higher than those of the untreated fiber. The work of adhesion between fiber and matrix was not directly proportional to IFSS for both the untreated and ED-treated cases. The matrix toughness might contribute to IFSS more Likely than the surface wettability. Interfacial properties of the epoxy-PEI composite can be affected efficiently by both the control of matrix toughness and ED treatment.