Abstract: The effect of the hot deformation on the grain size of SA508 Gr.3Cl.1 steel after forging and the corresponding relation between the grain size after forging and the grain size after normalizing were studied by plane compression deformation test. The results show that the temperature and deformation have a great influence on the grain size after forging,while it have little effect on the grain size after normalizing. The grain size after normalizing is fine and uniform, which is about 20 μm. According to the response relationship between normalized grain size and grain size after forging, SA508 Gr.3Cl.1 steel has little microstructure heredity, which means that the grain size after normalizing is not sensitive to one after forging. Thus, from the point of grain size, the focus of the SA508 Gr.3Cl.1 steel forging process should be on the part shape and size, which has significance for reducing its forging difficulty for forging process of complex and large forgings.