Abstract: The effect of final temperature of ultra fast cooling after rolling on microstructure and yield ratio was studied in laboratory by a φ 450 mm hot-rolling mill and an ultra fast cooling system. The results show that the strength and yield ratio of the tested steel increased when final cooling temperature varies from 630 ℃ to 360 ℃, and microstructure passed through the transformation of composite phases microstructure including bainite ferrite matrix and M/A island phase to the single lath bainite microstructure, and the yield ratio increased to 0.94 from 0.69. When final cooling temperature was about 550 ℃, the yield strength was 650 MPa, the tensile strength was 955 MPa, the yield ratio was 0.68, and the elongation was about 19.2%.