Polycyanurates generated from polycyclotrimerization of bisphenol A dicyanate (BPADCy) were blended with either hydroxyl-terminated or cyanated poly(ether sulfone) (as HPES or CPES, respectively) to improve their mechanical properties. Results from scanning electron microscopy suggest a discrete-continuous fracture surface for the polycyanurate blended with 10 wt% of HPES; but for the CPES-blended polycyanurate, a blurring interface between particle and matrix was observed. The blurring interface was originated from inter-reactions through the cyanate groups in CPES and BPADCy. This blurring interface can also be generated if 1 wt% of catalyst system (n-nonylphenol/cobaltic acetylacetonate) was used during cure of mixtures of HPES and BPADCy. This blurring interface in the catalyzed system is attributed to the chemical reaction between the hydroxyl termini in HPES and the cyanate groups in BPADCy. The discrete particle sizes in the cured HPES/BPADCy strongly depend on the curing temperatures. With a higher curing temperature, a larger average particle size was generated.