Abstract: To reveal the sensitivity of strength and fracture toughness to loading rate for ultra-strength steel 18NiC250, the plastic flow stress-strain curves of 18NiC250 were measured in the strain-rate range from 0.001 s-1 to 2 000 s-1 and its fracture toughness was obtained in the loading-rate range from 10-1 MPam1/2/s to 106 MPam1/2/s by using a CSS4410 test machine and a SHPB system. And the fracture failure mechanism of 18NiC250 was discussed by fractography and Taylor experiment. Results show that the flow stress of 18NiC250 is not sensitive to loading rate and it can remain 1.9 GPa. But the fracture toughness of 18NiC250 is quite sensitive to loading rate, it decreases by 38.2% when the loading rate increases from 10-1 MPam1/2/s to 106 MPam1/2/s, and the corresponding fracture mode changes from ductile dimple to cleavage.