Metallographic and fractographic tests of liquid metal embrittlement are performed for nanocrystalline Ni-Hg systems. It is shown that the behaviour of nanocrystalline nickel under these conditions is close to that of ordinary polycrystalline materials. The presence of a stage of subcritical crack growth is demonstrated. As nanocrystalline grains have none of their own intrinsic dislocations, it is assumed that subcritical crack growth in liquid metal environment can be realized through the mechanism of dissolution of atoms from the crack tip. This dissolution-condensation model of liquid metal embrittlement, developed for polycrystals, can also be applied to nanocrystals.