Abstract: Epoxy resins with highly cross linked network and superior performance such as excellent mechanical, thermal and electrical properties, outstanding chemical and moisture resistance, are widely used in the automotive and aerospace industries. In this work, Diamino diphenyl sulfone (DDS) cured diglycidyl ether of bisphenol-A (E-51) was modified by a highly soluble polyamide acid oligomer (PAA-n). PAA-n was synthesized via the reaction of polyphthalic acid anhydride and aromatic diamine containing tert-butyl groups. Results obtained by FT-IR,¹H-NMR, DSC and ESI confirm that PAA-n with molecular weight of 839 and high purity was obtained. UV/vis shows that the solution composed of PAA-n and E-51 follows Beer law, which indicates there is a good compatibility between PAA-n and E-51. Bulky tertiary butyl on benzene ring of PAA-n may prevent close packing of oligomer chains by increasing free volume of polymer, and the space location-obstruct effect hamper polymer chains close to each other so as to reduce effectively the interaction between polymer chains of PAA-n. Thus, without using any organic solvent, 0~2% (mass fraction) PAA-n are added into E-51. Using DDS as a curing agent, the curing kinetics of E-51/PAA-n composites is studied. By SEM, E-51/PAA-n composites are composed a continuous phase, and no phase separation can be observed, due to the high chemical reactivity between amino or carbonoxylic groups of PAA-n and epoxy resin. The impact strength of composites can be increased upon increasing the amount of PAA-n, and reaches 42 kJ/m² for composites containing 2% PAA-n (EP-PAA-2), which increases 72.2% compared with pure E-51. At the same time, the tensile and flexural strengths can be almost maintained, while their moduli are increased. DSC and DMA results reveal that the glass transition temperature (T_g) is increased gradually from 196 ℃ for pure E-51 to 219 ℃ for EP-PAA-2 with 3% decrease in storage modulus.