Abstract: The morphology of lower bainite formed by isothermal quenching of 40Cr steel was observed by optical microscope, scanning electron microscope, electron backscatter diffraction and transmission electron microscope, and the differences with martensite and tempered sorbite were analyzed. The results show that the morphology of lower bainite was obviously different from that of martensite and tempered sorbite, and the hardness of lower bainite was higher than that of tempered sorbate. The needle like structure of lower bainite was densely arranged in bundles, with strong sense of direction and color after erosion. The inside of the structure was fine carbide precipitated on the ferrite lath. The carbides were arranged in order, and the included angle with the long axis of ferrite was 60°.