Abstract: A Mn-Mo-Nb (Ti,V) construction steel (WGJ510C2) has such properties as lower ratio of σs/σb (≤0.8),refractoriness,weather resistance and high heat input weld-ability. Its fracture behavior was investigated because of the most important usage in construction. The samples were cut from the hot rolled sheet with dimensions of 20 mm×40 mm×180 mm for examining the fracture toughness of the steel by means of three-point-bend test. The test was performed to get elastic-plastic fracture mechanical parameters (CTOD): δR (critical value of opening displacement of the rapture),Δa (propagation value of static rupture) and JR (crack propagation resistance). The crack propagation behaviors can be estimated from the analytic fracture model which show that the parameters have following regression relationship,i.e.,δR=0.787(0.123+Δa)0.685;or JR=461.2Δa0.435. The high value of δR indicated that the plastic store energy at crack tips is high. By calculating,δc＝0.187 mm,J0.2=229 kJ/m2 at 0 ℃. It can also be concluded that fracture resistance of the steel is excellent.