A theoretical model of plastic deformation of nanocrystalline ceramics in the region of small grain sizes is proposed. In the framework of the model, deformation is described as the combined action of grain boundary sliding and emission of lattice dislocations from triple junctions of grain boundaries. Using the method of discrete dislocation dynamics we obtained an inverse Hall-Petch relation, which qualitatively coincides with the experimentally measured dependences of microhardness in nanocrystalline ceramics with extremely small grain sizes.