To analyze the good toughness of ultrafine ferrite/cementite steels, the concept of effective grain size (d_EFF) is applied to ductile-to-brittle transition temperature, DBTT, for ultrafine ferrite/cementite (Uf-F/C), ferrite/pearlite (F/P), quenched (Q), and quench-and-tempered (QT) microstructures in a low carbon steel. The d_EFF is determined to be 8, 20, 100, and 25 μm for Uf-F/C, F/P, Q, and QT, respectively. In F/P and Q, it is in accordance with the ferrite grain size and the prior austenite grain size, respectively. In QT, the d_EFF fits the martensite packet size. In Uf-F/C, the ferrite grain size has a bimodal distribution and the larger grain size corresponds to the d_EFF, which is the smallest among the four microstructures. In terms of the relationship between d_EFF and DBTT, the Uf-F/C, Q, and QT microstructures can be placed into the same group and the F/P to a different one. Furthermore, the Uf-F/C has the highest estimated fracture stress among the four microstructures. These might be the result of the difference in the surface energy of fracture, namely the former is estimated to have a surface energy of 34.6 J/m² and latter a surface energy of 7.7 J/m². Thus, the excellent toughness of the ultrafine ferrite/cementite steel can be attributed to the small d_EFF and the high surface energy of fracture.