Stiff and brittle cyanate ester (CE) resin was modified by copolymerizing it with epoxy resin (ER) and phenolic resin (PR) to improve its toughness and flexibility. The cure process of the modified CE resin was characterized by gel time curves and differential scanning calorimetry curves. The Fourier transform infrared spectra of the modified CE resin showed its chemical structure during the curing process. The mechanical properties, thermal behavior, dielectric properties, and morphology of the modified CE resins were investigated. The results showed that an increase in epoxy and phenolic resins resulted in improved flexibility while maintaining thermal stability. When the mass ratio of CE/ER/PR was 70: 15 :15 (w/w), flexural strength and impact strength of the modified CE resin increased from 113.6 MPa and 5.2 kJ/m(2) to 134.5 MPa and 16.7 kJ/m(2), respectively. Little of the thermal stability and dielectric properties was sacrificed in the modification of the CE. (c) 2006 Wiley Periodicals, Inc.