Experimental studies were conducted on the dynamic fracture behavior of AISI 5140 steel over a wide range of loading rates. True stress-strain relations of the steel were measured under different strain rates, and a dynamic constitutive model was then expressed with the strain-hardening term and the strain-rate hardening term. Fracture tests were investigated under quasi-static condition, instrumented Charpy impact condition and Hopkinson pressure bar impact condition. Fracture toughnesses corresponding to each test condition were also determined. Fracture characteristics were analyzed with observations of the fracture appearances. It was found that the fracture toughness decreased significantly with the increasing loading rate, and the fracture mechanisms of the steel at various loading rates were brittle fractures characterized by river markings and secondary cracks. Based on the fracture assessment method of the CEGB R6 procedure, the effects of the strain rate and the loading rate on the assessment curve were discussed.