The safety of aged steel structures, especially those with cracking members, has aroused a great deal of attention. In this paper, the application of fracture mechanics for evaluating the ultimate load-carrying of steel material is proposed. Three-point bending tests of three specimens with the mode-I fracture are done to get the ultimate load-carrying capacity, when specimens are destroyed. Fracture ductility KIC, and the steel allowable stress [σ] with certain crack length are formulated. Magnetic method for measuring cracks of steel members is adopted for an aged steel truss bridge, and material composition of specimens is tested, and field nondestructive load test involved static and dynamic test are carried into execution. By comparing the results of the calculation and those of the field test, some conclusions are obtained, which are likely to contribute to safety evaluation of aged steel structures, or contribute to designing similar bridges.