Poly(propylene carbonate) (PPC) was used as a toughening agent for improving the brittleness of cured epoxy resins (EP). Methyl tetrahydrophthatic anhydride (MTHPA) was used as a curing agent. The activation energies for the reactions of PPC/MTHPA and EP/MTHPA measured by FTIR were 115.8 and 66.5 kJ/mol, respectively, while for the composite system of PPC/EP/MTHPA, the activation energy obtained from DSC was 52.9 kJ/mol. Gel contents, DMA, and DSC displayed that the cured resins of PPC/EP/MTHPA were phase-separation crosslinking systems and most of PPC could react with MTHPA or the epoxy group. The toughness of cured resins was reinforced by the addition of PPC. The optimum mechanical properties and toughness for cured resins of PPC/EP/MTHPA corresponded to the system containing 20 phr PPC, which achieved a 33% increase in tensile strength and a 45% increase in the fracture toughness at no expense of the elongation of cured resins.