Abstract: Epoxy resin (EP) was widely used in electrical and electronic fields because of low curing shrinkage rate, dimensional stability and outstanding electrical insulating performance. However, the poor toughness and low dielectric constant limit its further applications in dielectric fields. Herein, using methyl nadic anhydride (MNA) as curing agent, polyether sulfone (PES) and nano-alumina (Al₂O₃) as reinforced fillers, a new kind of epoxy resin/polyether sulfone/nano-alumina (EP/PES/Al₂O₃) composite was fabricated by a simple blending-curing process. The FE-SEM photos of EP/PES show that PES displays a spherical structure with a diameter of 2-4 μm and is uniformly distributed in EP matrix. As shown in EP/PES/Al₂O₃ composite photo, the further introduction of Al₂O₃ patched many holes between PES particle and EP matrix, which may greatly improve the performance of composite materials. Their mechanical, thermal, and dielectric properties were determined by universal tensile testing machine, thermal gravimetric analyzer and broadband dielectric spectrometer. The results show that when the mass fractions of PES and nano-Al₂O₃ are 15% and 3%, the impact strength, tensile strength, and bending strength of EP/15PES/3Al₂O₃ composites reach 45.7 kJ/m², 87 MPa and 180 MPa, respectively, indicating a significant improvement compared to pure epoxy resin (14.7 kJ/m², 85 MPa and 127 MPa). In addition, the dielectric constant and dielectric loss of the EP/15PES/3Al₂O₃ composites were 7.7 and 0.013 5 at room temperature (with a measurement frequency of 100 Hz), which were higher than those of the pure epoxy resin with a dielectric constant of 4.5 and dielectric loss of 0.006 9. Moreover, the thermal properties of EP/PES/Al₂O₃ composites were improved significantly because of the excellent thermostability of PES and Al₂O₃. For the sample of EP/15PES/3Al₂O₃, the initial decomposition temperature was improved by 73℃ compared to the pure epoxy resin, indicating the improvement in thermal stability of EP/PES/Al₂O₃ composites.