Hydroxyl-terminated polybutadiene (HTPB) rubber was used as a modifier for bisphenol A dicyanate ester (BADCy) to improve its toughness. The effect of catalyst on the curing behavior of the resin and the curing techniques were developed by differential scanning calorimetry. The dynamic mechanical analysis, thermogravimetry analysis, and tensile test were used to study the thermal behavior, mechanical properties, and dynamic mechanical performance of the modified BADCy resins. Results show that addition of HTPB resulted in improved toughness while maintaining relatively good thermal stability. Additional, phase separation phenomenon was confirmed by the analysis of scanning electron microscope. For the toughening of the resin, the main toughening mechanism is shear yielding and crack branching. POLYM. ENG. SCI., 51:1404-1408, 2011. (C) 2011 Society of Plastics Engineers