A theoretical model is suggested that describes a mechanism of plastic deformation in high-temperature ceramic materials containing amorphous intercrystalline layers (AILs) and pores in triple junctions of AILs. Within the model, the plastic deformation is realized through the generation of liquid-like inclusions on pore surfaces and their subsequent propagation along the AILs. In the exemplary case of high-temperature α-Al2O3 ceramics with AILs, the dependences of the critical values of the external shear stress for the formation of a liquid-like inclusion on deformation temperature in a wide range of the deformation temperatures from 300 to 1500 K are calculated. It is shown that the critical stress for the nucleation of a liquid-like inclusion strongly depends on the deformation temperature and weakly depends on the pore size.