A series of imidazole (MI) blocked 2,4-toluene diisocyanate (TDI) with polyethylene glycol (PEG-400) as soft segment (PEG-MI-b-TDI) were synthesized for toughening and curing the bisphenol A type epoxy resin (E-44). Fourier transform infrared (FTIR) spectrum indicates that the NCO groups of the isocyanate molecule are blocked with MI. For curing epoxy systems, elimination of epoxy group and the formation of urethane bonds were studied by FTIR spectroscopy. The results of mechanical property were shown that the tensile shear and impact strengths of neat MI and MI-b-TDI cured E-44 are lower than those of PEG-MI-b-TDI cured E-44. Based on the scanning electron microscope studies, microstructure evolutions of the E-44 cured by different curing agents were imaged. The mechanical, thermal, and dynamic mechanical properties were measured by universal testing machine, differential scanning calorimeter and dynamic mechanical analyzer (DMA). The toughness of E-44 cured by PEG-MI-b-TDI was effectively improved without sacrificing the tensile shear strength. Based on the DMA studies, the long soft chain of PEG brought in a noticeable lowering in the glass transition temperature (Tg). The glass transition temperature is near 165°C for the neat MI cured E-44, which is higher than the Tgs of the other curing agents cured epoxy. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41345.