The toughening of epoxy modified with poly(butylene terephthalate)/poly(tetramethylene glycol) (PBT-PTMG) copolymers of various chemical composition was investigated. The fracture toughness of the brittle epoxy was highly enhanced by the inclusion of PBT-PTMG copolymer without loss of other intrinsic mechanical properties, such as modulus and yield stress. These modified epoxies also exhibited synergism in toughening. The remarkable enhancement and the synergism in fracture toughness of PBT/PTMG-modified epoxies is possibly due to the enhancement of the degree of phase transformation toughening, which is a result of the enhancement of the degree of perfectness of PBT spherulites in the presence of PTMG segments. The changes in micro-morphology of PBT/PTMG phases induced by the different chemical composition of copolymer is the most important cause of the dependency of the fracture energy on the processing variables, such as the relative PBT/PTMG composition and total amount of modifiers. Other toughening mechanisms, such as crack bifurcation, ductile fracture of PBT/PTMG phases, main crack-path alteration, and crack bridging, also contributed to toughness enhancement of the modified epoxies.