Abstract: In order to analyze the microstructure unit controlling the toughness of 0.02C-5Mn low-carbon lath martensite steel, the effects of high angle boundaries on the propagation of impact fracture cleavage cracks or the propagation of secondary cleavage cracks near the impact fracture in the steel were studied by electron backscatter diffraction (EBSD). Further more, based on the misorientation of ideal cleavage planes 100 between martensitic variants, the ability of packet and block boundaries to change the propagation direction of cleavage cracks was compared. The results indicate that the cleavage cracks or secondary cleavage cracks were deflected through large angles when crossing through the prior austenite grain boundaries, packet boundaries and most of the block boundaries. The ability of packet and block boundaries to hinder cleavage crack propagation was close. Blocks are the microstructure unit controlling cleavage fracture and ductile to brittle transition temperature of lath martensite.