The enhancement of toughness at low temperatures in fine-grained low carbon steel was studied, basing on the theory of crack-tip shielding due to dislocations. Low carbon steel was subjected to an accumulative roll bonding (ARB) process for grain refining. The grain size perpendicular to the normal direction was decreased to approximately 200nm after the ARB process. The fracture toughness of low carbon steel with the ARB process was measured at 77K by four-point bending, comparing with the fracture toughness of those without the ARB. It was found that the value of fracture toughness at 77K was increased by grain refining due to the ARB process, indicating that the ARB process enhances toughness at low temperatures and that the brittle-to-ductile transition (BDT) temperature shifted to a lower temperature. Quasi-two-dimensional simulations of dislocation dynamics, taking into account crack tip shielding due to dislocations, were performed to investigate the effect of a dislocation source spacing along a crack front on the BDT. The simulation indicates that the BDT temperature is decreased by decreasing the dislocation source spacing.