We examined the ring-opening copolymerization of glycidyl phenyl ether (GPE) and d,l-lactide (LAC) to afford poly(GPE-co-LAC) as a model reaction of the LAC-mediated thermal curing of epoxy resins such as 2,2-bis(4-glycidyloxyphenyl)propane (BGP), 2,2-bis(4-glycidyloxyphenyl)methane (BGM), 3,3′,5,5′-tetramethyl-4,4′-diglycidyloxybiphenyl (TDP), N,N-diglycidyl-4-glycidyloxyaniline (DGA), and oligo phenols (NC-3000 and EPPN-502H). The reaction of GPE and LAC proceeded efficiently in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) at 180 °C for 2 h. The monomer reactivities r₁ of LAC and r₂ of GPE calculated according to the Fineman-Ross method and the Kelen-Tüdӧs method were 0.85 and 1.26, respectively. The relatively small difference between these values suggests that the produced poly(GPE-co-LAC) would be a gradient copolymer, different from a block copolymer or random copolymer. Based on these results, we examined the thermal curing reactions of epoxy resins BGP, BGM, TDP, DGA, NC-3000, and EPPN-502H in bulk under the same conditions. The corresponding cured products were obtained quantitatively. The thermal stability of BGP, BGM, DGA, and EPPN-502H was increased by the addition of a small amount of LAC, suggesting that the cross-linking density of the cured products was increased. This new class of thermal curing system should be applicable to epoxy resins used in various industrial applications.