The axial temperature distribution in the sillenite fibers was calculated based on the elastic properties of the material. The dislocation density was evaluated by the Tsivinskii's relation and compared with obtained by the chemical etching method on single-crystal Bi 12SiO 20 fibers grown the LHPG method. It was shown that BSO fibers will crack due high axial temperature gradients if the fiber diameter exceeds some critical value D crit. The relation between the temperature gradients and the structural perfection of the as-grown fibers and the ways of the optimization of the temperature distribution during the fiber growth process has been discussed.