Abstract: A commercial available amphiphilic triblock copolymer poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG-PPG-PEG) was utilized to toughen and reinforce aerospace-grade epoxy thermosets (diglycidyl 4, 5-epoxycyclohexane-1, 2-dicarboxylate, TDE-85) cured with 4-aminophenyl sulfone (DDS). It was found that upon an optimal PEG-PPG-PEG loading of 1 wt.%, the tensile strength, tensile elongation at break and critical strain energy release rate (G1c) increased by about 24.2%, 59.3% and 125%, respectively, without significantly sacrificing the modulus and thermal stability of the thermosets. Transmission electron microscopy and scanning electron microscopy results evidenced the changes in the mechanical properties observed may be correlated with the self-assembled structure and phase structure of PEG-PPG-PEG formed within the epoxy matrix. Within the copolymer loading investigated (0~5 wt.%), PEG-PPG-PEG formed nanoscale vesical micelles consisting a PPG core and PEG corona with 40~90 nm in the size. Moreover, large copolymer domains (0.2~1.2 μm in size) also appeared with increasing copolymer loading. It is believed that the coexistence of nanoscale and macroscale phase structure may be responsible for the simultaneous toughening and reinforcing effect of amphiphilic triblock copolymer towards the high performance epoxy investigated.