The double-torsion test was employed to study the processes of crack propagation and to measure the fracture toughness of polycrystalline diamond. The value of fracture toughness of about 13 MPam(1/2) is surprisingly high. Inhomogeneity in microstructure may cause discontinuous crack propagation which makes it difficult to study the subcritical crack growth behaviour of this polycrystalline material. Subcritical crack growth is shown to be negligible and crack deflection is shown to be an important toughening mechanism in polycrystalline diamond.