In order to investigate roles of grain boundaries on the improved fracture toughness in ultrafine-grained metals, interactions between crack tips, dislocations, and disclination dipoles at grain boundaries are performed to aluminium bicrystal models containing a crack and <112> tilt grain boundaries using molecular dynamics simulations. A proposed mechanism to express the improved fracture toughness in ultrafine-grained metals is the disclination shielding effect on the crack tip mechanical field. The disclination shielding can be activated when a transition of dislocation sources from crack tips to grain boundaries and a transition of the grain boundary structure into a neighbouring energetically stable boundary by emitting dislocations from the grain boundary occur. The disclination shielding effect becomes large as dislocations are continuously emitted from the grain boundary without dislocation emissions from crack tips. This mechanism can further shield the mechanical field around the crack tip and obtain the plastic deformation by dislocation emissions from grain boundaries, hence it can be expected that the disclination shielding effect can improve the fracture toughness in ultrafine-grained metals.