Abstract: Effects of secondary quenching temperature on microstructure and yield ratio of HSLA (high strength low alloy) 100 steel in the two-phase region were investigated. The results show that the mixed microstructure of ferrite and the M-A(martensite-austenite)islands were obtained in the two-phase region after secondary quenching. A small amount of island structure distributing along the lath ferrite was observed when quenching temperature was 700 ℃. With the increase of quenching temperature the island structure increased in the number and coarsened in the shape, and the ferrite morphology gradually changed from a single lath to the multilateral shape with the decrease of the dislocation density. When quenching temperature was 820 ℃, the microstructure reverted to lath bainite. The yield ratio of the test steel continually declined from 0.94 to 0.67 in the range of quenching temperature from 700 ℃ to 740 ℃, but kept a very small fluctuation in the range of 740-780 ℃ (about 0.67), and finally raised sharply at quenching temperatures above 780 ℃.