We investigated the interaction of a brittle crack with low and high angle grain boundaries in BCC Iron by means of atomistic simulations at finite temperatures. It is demonstrated that both low and high angle grain boundaries exhibit resistance to brittle crack propagation. The resistance is controlled by the ability of a grain boundary interface to structurally transform, which can involve grain boundary sliding or the emission of misfit dislocations. Here, we observed deformation mechanisms which generally correspond to those reported in experiments for Fe–Si polycrystals. We show that low and high angle grain boundaries exhibit different intensities of plastic deformation upon interaction with a brittle crack, thereby offering different resistance to its propagation.