A model describing both direct and inverse Hall-Petch dependences observed in nanocrystalline ceramic MgAl₂O₄ spinel is proposed. Within the model, plastic deformation in nanocrystalline ceramics (NCCs) is realized via lattice dislocation slip combined with thermally activated grain boundary (GB) sliding. The model strongly suggests that the controlling parameter determining the type (direct or inverse) of the Hall-Petch dependence is the GB sliding activation energy. It is assumed that this quantity can be affected by the temperature regime of NCC synthesis and therefore rationalize conflicting data reported in the literature concerning the onset of the inverse Hall-Petch behavior in this system.