Ultra-fine grained steels were recently developed by thermo-mechanical warm rolling. Their low-temperature fracture toughness was evaluated in terms of crack tip opening displacement (CTOD) in this paper. Effect of temperature on CTOD and the correlation between CTOD and grain size were investigated, and the experimental results showed that refining ferrite grains can increase the fracture toughness of steel and lower the sensitivity of fracture toughness to temperature. The fracture toughness of the developed ultra-fine grained steels was superior to that of hot-rolled steel SM490B with similar chemical composition.