Both fibril and spherical polyaniline (PANI) nanostructures have successfullyserved as nanofillers for obtaining epoxy resin polymer nanocomposites (PNCs).The effects of nanofiller morphology and loading level on the mechanicalproperties, rheological behaviors, thermal stability, flame retardancy,electrical conductivity, and dielectric properties were systematically studied.The introduction of the PANI nanofillers was found to reduce the heat-releaserate and to increase the char residue of epoxy resin. A reduced viscosity wasobserved in both types of PANI–epoxy resin liquid nanosuspension samples atlower loadings (1.0 wt?% for PANI nanospheres; 1.0 and 3.0 wt?% for PANInanofibers), the viscosity was increased with further increases in the PANIloading for both morphologies. The dynamic storage and loss modulii werestudied, together with the glass-transition temperature (Tg)being obtained from the peak of tan δ. The critical PANI nanofiller loading forthe modulus and Tg was different, i.e., 1.0 wt?% for thenanofibers and 5.0 wt?% for the nanospheres. The percolation thresholds of thePANI nanostructures were identified with the dynamic mechanical property andelectrical conductivity, and, because of the higher aspect ratio, nanofibersreached the percolation threshold at a lower loading (3.0 wt?%) than the PANInanospheres (5.0 wt?%). The PANI nanofillers could increase the electricalconductivity, and, at the same loading, the epoxy nanocomposites with the PANInanofibers showed lower volume resistivity than the nanocomposites with the PANInanospheres, which were discussed with the contact resistance and percolationthreshold. The tensile test indicated an improved tensile strength of the epoxymatrix with the introduction of the PANI nanospheres at a lower loading (1.0wt?%). Compared with pure epoxy, the elasticity modulus was increased for allthe PNC samples. Moreover, further studies on the fracture surface revealed anenhanced toughness. Finally, the real permittivity was observed to increase withincreasing the PANI loading, and the enhanced permittivity was analyzed by theinterfacial polarization.