Liquid-crystalline (LC) epoxy resins were cured at different temperatures to obtain polydomain LC phase-cured resins. The cured resins had polydomain structures with a nematic LC phase and their domain diameters differed depending on the curing temperatures. The relationship between the domain diameter and fracture toughness of the diglycidyl ether of terephthalylidene-bis-(4-amino-3-methylphenol) (DGETAM)/m-phenylenediamine (m-PDA) systems with the nematic phase and the previously reported smectic LC phase structures was investigated. It was clarified that the highly ordered LC structure (smectic phase) in each domain could improve the fracture toughness. In addition, the changes in the network orientation of the DGETAM/m-PDA systems were evaluated by a mapping of the microscopic infrared dichroism in the fracture process and their toughening mechanism was suggested. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2337-2345, 2010