Polycyanurate derived from bisphenol A dicyanate (BPADCy) is brittle in nature; therefore, different amounts of hydroxyl- or cyanate- terminated poly(ether sulfone) (as HPES or CPES) was mixed and heated with BPADCy to obtain a thermoplastic-blended polycyanurate. Morphology of this thermoplastic-blended polycyanurates was investigated by SEM, and their fracture toughness was evaluated by the propagation of the deformation zone after immersion of the sample film in methylene chloride. The results suggest that polycyanurate blended with 50 wt% of HPES (or CPES) has higher fracture toughness than sample toughened with 10 wt% of HPES (or CPES). In addition, polycyanurates blended with CPES usually have higher fracture toughness than those blended with same amounts of HPES. These results can be qualitatively correlated with the morphology difference observed from the SEM study. (C) 2000 Elsevier Science Ltd. All rights reserved.