Theoretical model is suggested which describes the micromechanism of lattice dislocations emission from triple junctions of amorphous intercrystalline layers with pores and liquid-like inclusions in high-temperature ceramics. Within the model, the plastic deformation in ceramics under elevated temperatures is realized through the emission of lattice dislocations from triple junctions of grain boundaries and the subsequent glide of the emitted dislocations in the grain interior. In the exemplary case of high-temperature α-Al2O3 ceramics, a comparative analysis of the critical stresses for the emission of the lattice dislocations, and for their glide along the prismatic and basal slip planes in the grain interior depending on the deformation temperature was carried out in a wide temperature range from 300 to 1500 K. It is shown that the critical stress for the emission of the lattice dislocations decreases with both an increase in the length of the liquid-like inclusion and the deformation temperature, and increases with increasing the pore size.