Abstract: The microstructure characteristics of F40 type hull structure steel and the effect of microstructure on its low-temperature toughness were studied by OM, SEM, TEM, and EBSD. The mechanism of low-temperature toughness was also analyzed. It is shown that the micro-alloyed steel possesses high strength and excellent low-temperature toughness due to a mixed microstructure of acicular ferrite and quasi-polygonal ferrite. The grain boundaries between the two microstructures and the grain boundaries between acicular ferrites are the large boundary, which is pronounced to inhibit cleavage crack propagation and improve the crack propagation energy. The mixed microstructure shows the capability to attain such an excellent low-temperature toughness that the Charpy-V impact energy can reach 138 J at -80℃.