The effect of high temperature predeformation on deformation characteristics of ductile LiF and brittle BaF, crystals was investigated. The same predeformation, as has been shown previously, leads to the increasing of the limited plastic deformation of MgO crystals by one order of magnitude. It was found that in both investigated crystals, as in the case of MgO crystals, at the test temperature T, 0.1 T, limited plastic deformation before fracture obviously increases if the speoimen was prestrained up to cp 4 1 yo at TI = 0.5 Tm. This increase is as higher as the plasticity of crystal is lower. I n BaF, crystals it reaches a factor of 20 and more. The effect is connected with the creation of mobile dislocations during the high temperature predeformation and dislocation sources for subsequent deformation a t lower temperatures and with the homogeneity of the process of plastic deformation. These conditions prevent the appearance of dangerous places in which the fracture can begin.