The brittle-to-duetile transition (BDT) behaviour was investigated in low carbon steel deformed by an accumulative roll-bonding (ARB) process. The temperature dependence of fracture toughness was measured by conducting four-point bending tests at various strain rates. The fracture toughness increased while the BDT temperature decreased in the specimens deformed by the ARB process. Arrhenius plots between the BDT temperatures and the strain rates indicated that the activation energy for the BDT did not change due to the deformation with the ARB process. It indicated that the decrease in the BDT temperature by grain refining was not due to the reduction in the dislocation mobility with respect to short-range barriers. Quasi-three-dimensional simulations of discrete dislocation dynamics indicated that the decrease in the number of dislocation sources decreases in the BDT temperature. The roles of grain boundaries will be also discussed in order to explain the decrease in the BDT temperature.