Abstract: Different molecular weights of tetra amine terminated polyether with urethane urea groups (TAPEU) were synthesized and characterized with FT IR and NMR. Then, TAPEUs were used as toughening agent for diglycidyl ether of bisphenol A epoxy resin (DGEBA)/diethylenetriamine (DETA) system. The degree of hydrogen bonding, glass transition temperature, network structure, tensile fracture morphology and mechanical properties of the modified epoxy resins were investigated. The results showed that with the increasing molecular weight of TAPEU in the toughened epoxy systems, the crosslinking density decreased, the degree of hydrogen bonding enhanced, and the glass transition temperature increased. Tensile fracture morphology became rough, the micro phase structure was observed, and toughness and impact resistance improved significantly when TAPEU was introduced into the epoxy resin. Compared with neat epoxy, the impact strength and elongation at break of the TAPEU2000 50 modified epoxy resin was enhanced by 65% and 330%, respectively.