Two maleimido-end-capped poly(ethylene glycol) (m-PEG)-modified bismaleimide (BMI) resins [4,4'-bismaleimido diphenylmethane (BDM)] were synthesized from poly(ethylene glycol) (PEG) of two different molecular weights. A series of m-PEGs and unmodified BDM were blended and thermally cured. The effect of incorporating m-PEG side chains on the morphology and mechanical behaviors of BMI polymer were evaluated. The mechanical properties of these m-PEG-modified BMls that were evaluated included flexural modulus, flexural strength, strain at break, fracture toughness, and fracture energy. The morphology of these blends was studied with scanning electron microscopy. All the m-PEG-modified BMI polymers showed various degrees of phase separation: depending on the molecular weights and concentrations of the m-PEG used. The effects of these morphological changes in the m-PEG-modified BMI polymers were reflected by the, improved fracture toughness and strain at break. However,, there was a reduction in the flexural moduli in all m-PEG-modified BMI polymers.