Abstract: The effect of finishing rolling temperature (880 〜935℃) on the actual grain size of hot rolled medium carbon boron steel 40B has been studied. With start rolling at 990 〜1010℃ , the different finishing rolling temperatures are achieved by controlling the rolling rhythm of round steel. It is verified that decreasing finishing rolling temperature is beneficial to improve the actual grain size Φ32 mm hot rolled medium carbon boron steel. Through experimental production and analysis of the longitudinal metallographic structure in the radius 1/2 of the head, middle and tail of round steel, it is found that the actual grain size of hot rolled medium carbon boron steel is the smallest and uniform as the finishing rolling temperature is in the range of 880 〜890℃.