The characteristics and mechanical properties of the (alpha+theta) microduplex structures formed by various thermomechanical processings were studied in an ultrahigh carbon steel (Fe-1.4Cr-1.0C). When the pearlite was austenitized in the (gamma+theta) region, quenched and tempered at the temperature below A(1) an (alpha+theta) microduplex structure in which most of alpha boundaries are of high-angle one is formed through the recovery of the fine (lath martensite + undissolved theta) mixture during tempering. Such (alpha+theta) microduplex structure with high-angle a boundaries exhibits superior superplasticity compared with those formed by the heavy warm rolling of pearlite, or by the annealing of heavily cold-rolled pearlite which contain higher fraction of low-angle alpha grain boundaries. Although the microduplex structures obtained various thermomechanical processings exhibit nearly the same tensile strength (about 900MPa), the ductility is markedly influenced by the nature of alpha grain boundaries. The specimens which contain large portion of high-angle alpha grain boundaries exhibit better ductility such as 25% in total elongation.