Epoxy resin was modified with polyurethane having different isocyanate index. Impact strength, tensile strength, elongation at break and flexural strength were estimated as functions of polyurethane content and isocyanate index. Hardness, linear expansion coefficient and the deflection temperature under load were estimated for selected compositions. Infrared spectroscopy was carried out for unmodified epoxy resin and compositions with improved mechanical properties. Maximum improvement in the fracture toughness was reached with polyurethane with highest content of isocyanate. Hardness was not affected but elastic modulus decreased, implying a softening of epoxy-based compositions. The infrared spectra indicated that an excess of isocyanate groups leads to a grafting process between the modifier and the matrix, explaining the toughening of the latter.