A bstractLarge-angle grain boundaries in steel o er an important resistance to the propagation of cleavage cracks that a ects the fracture toughness and can modulate the ductile-to-brittle transition temperature of fracture downwards. This behaviour has been studied in bicrystals of Fe±3 wt% Si alloy in detail. It was noted that the twist misorientation across a large-angle boundary has a more profound e ect on cleavage fracture resistance than the tilt misorientation does. Speci®c measurements of such resistance at ¡ 20°C over a random selection of large-angle grain boundaries in bicrystals and associated fractographic studies have led to quantitative models of this resistance. The study has also revealed a transition from pure cleavage to mixed cleavage at around 0°C for this alloy, above which the observed increment of fracture work could be associated with the sigmoidal plastic bending and rupture of ligaments left between separate primary cleavage strips in the adjoining grain.