Poly(ethylene phthalate) (PEP) and poly(ethylene phthalate-co-ehtylene terephthalate) were used to improve the brittleness of the cycloaliphatic epoxy resin 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexane carboxylato (Celoxide 2021(TM)), cured with methyl hexahydrophthalic anhydride. The aromatic polyesters used were soluble in the epoxy resin without solvents and effective as modifiers for toughening the cured epoxy resin. For example, the inclusion of 20 wt % PEP (MW, 7400) led to a 130% increase in the fracture toughness (K-JC) of the cured rosin with no loss of mechanical and thermal properties. The toughening mechanism is discussed in terms of the morphological and dynamic viscoelastic behaviors of the modified epoxy resin system.