A bimodal-sized microstructure lied ill Ultrafine-grained ferrite/cementite steel fabricated through caliber-warm-rolling followed by annealing. The ferrite grain size distribution had a bimodal distribution Consisting of two peaks, a large peak and a small peak. A distinct absorbed energy transition or ductile-to-brittle transition occurred in Charpy impact tests. The ductile-to-brittle transition was due to the intrusion of cleavage. The transition shifted to higher temperature region with an increase in average gain size for the larger grain peak, with a simultaneous change in effective grain size on fracture Surface. The effective grain size apparently corresponded to the grain size of a larger grain peak of the bimodal,rain size distribution. More precisely, the effective grain size was found to better correspond to the larger peak of the bimodal distribution of grains with {100} planes almost parallel to the fracture plane. Fracture stress estimated became higher with a decrease in effective grain size. Namely the ultra-refinement of ferrite grain size could bring about the high fracture stress and contribute to an excellent toughness even for the bimodal-sized structure.