The fracture toughness of new type hot—rolled nano—scale precipitation strengthening steels (tensile strength of 700 MPa grade and 780 MPa grade) were evaluated by crack tip opening displacement (CTOD) experiments, and the influence mechanisms of microstructure, high angle grain boundaries, dislocation density and nano—scale precipitation on fracture toughness were discussed. The results indicated, when experimental temperature were room temperature, -10 and -30℃, the δ_Q0.2BL value of 700 MPa grade carriage strip were 0.468, 0.333 and 0.248 mm, and the δ_0.2 value of 700 MPa grade carriage strip were 0.298, 0.234 and 0.215 mm, respectively. However, the δ_Q0.2BL value of 780 MPa grade crossbeam strip were 0.311, 0.290 and 0.247 mm, and the δ_0.2 value of 780 MPa grade crossbeam strip were 0.212, 0.212 and 0.198 mm, respectively. Therefore, the fracture toughness of 700 MPa grade steel was better than 780 MPa grade steel. The differences of microstructure between 700 MPa grade steel and 780 MPa grade steel mainly included four aspects: (1) the microstructure of 700 MPa grade steel was mainly ferrite, while the microstructure of 780 MPa grade steel was mainly bainitic ferrite; (2) the carbide shape of 700 MPa grade steel was granular or short rod, and 780 MPa grade steel was strip carbide; (3) the dislocation density of 780 MPa grade steel was significantly higher than 700 MPa grade steel; (4) the proportion of large—angle grain boundaries of 700 MPa grade steel and 780 MPa grade steel were 85.6 % and 76.8%, respectively. Therefore, improving the volume fraction of ferrite and the proportion of high angle grain boundaries, refining carbide size and reducing dislocation density could effectively improve the fracture toughness of steels. Coarse precipitation (Nb, Ti)CN and grain boundary precipitation in microstructure deteriorated fracture toughness of steel, and semi—coherent precipitates nano—scale (Nb, Ti)C on ferrite or bainite matrix have less damaging effect on fracture toughness.