A semicrystalline polymer, isotactic poly(phenyl glycidyl ether) (i-PPGE) was used as a modifier for epoxy resin; 1,8-Diamino-p-methane (MNDA) and 4,4'-Diamino diphenyl sulfone (DDS) were used as curing agents. In the MNDA-cured resins, the dispersed phase were spherical particles with diameters in the range of 0.5-1.0 mum when the resin was blended with 5 phr i-PPGE. In the DDS-cured resins, the particle size distribution of the dispersed phase was much wider. The difference was traced back to the reactivity of the curing agent and the different regimes used for curing. Through dynamic mechanical analysis, it was found that in the MNDA-cured systems, i-PPGE had a lower crystallinity than in the DDS-cured system. In spite of the remarkable difference in the morphology and microstructure of the modified resins cured with these two curing agents, the toughening effects of i-PPGE were similar for these resins. The critical stress intensity factor (K-IC) was increased by 54% and 53%, respectively, for the resins cured by DDS and by MNDA, blending with 5 phr of the toughner. i-PPGE was comparable with the classical toughners carboxyl-terminated butadiene-acrylonitrile copolymers in effectiveness of toughening the epoxy resin. An advantage of i-PPGE was that the modulus and the glass-transition temperature of the resin were less affected. However, this modifier caused the flexural strength to decrease somewhat.