The effect of boron addition was investigated on the toughness of quenched steels whose carbon content extends from 0.2 to 0.8 percent. The results are as follows. (1) Impact values of boron added steels are higher than boron-free steels' over the above carbon range. (2) Boron increases effectively both crack initiation and propagation energies. (3) Typically in the case of 0.8% carbon, the dominant fracture mode is transgranular in the boron added steel, while it is intergranular in the boron-free. It suggests that the boron excludes phosphorous segregating at the grain boundaries and consequently it strengthens the grain boundaries.