Fracture toughness was studied in terms of crack-tip opening displacement (CTOD) in low-C medium-Mn high-strength steel at both room temperature and -40 degrees C, and excellent fracture toughness was obtained. The critical CTOD values () and crack extension (a) followed the relationship: =0.01343+0.62315a(0.47531) and =0.07391+0.48466a(0.60103) at room temperature and -40 degrees C, respectively. With the decrease in test temperature from room temperature to -40 degrees C, the corresponding when a=0.2mm ((0.2)) was reduced from 0.30341 to 0.25813mm, and intersecting point in the crack extension resistance curve with a 0.2-mm passivation line ((Q0.2BL)) was reduced from 0.42132 to 0.33941mm. The large fraction of high misorientation boundaries between tempered martensite effectively hindered the crack propagation and increased the fracture toughness. Furthermore, the submicron-scale complex laminated microstructure of tempered martensite and reversed austenite refined the effective fracture grain size, which inhibited crack propagation and led to high fracture toughness. Also, the excellent fracture toughness is attributed to strain-induced martensite transformation of reversed austenite in the small plastic deformation zone ahead of the crack tip, which absorbed the strain energy, relaxed the local stress concentration, suppressed the crack propagation, and enhanced the fracture toughness.