Abstract: The effects of quenching temperature and tempering temperature on the ratio of yield strength to tensile strength (yield ratio) in a medium carbon Cr-Ni-Mo ultra high strength steel have been investigated.After quenching and tempering at 200 ℃,the yield ratio first decreases and then increases with the increase of quenching temperature,being the lowest of about 0.45 at 950-1 000 ℃ quenching.When the quenching temperature was 900 ℃ and the tempering temperature was 165-180 ℃,the yield ratio was as low as 0.45 while the tensile strength was more than 2 200 MPa.The phenomenon of ultra low yield ratio correlated with the presence of retained austenite,large amounts of movable dislocations in retained austenite,and residual stress after quenching and tempering.