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The fibrillation of PBO fibers by physicochemical method was studied in this paper. The pretreatment on the fibers was processed by the etching solution under appropriate concentration in order to partly destroy the strong shell that is mainly composed by the amorphous macromolecules, then the fibers were swelled by the swelling solutions to reduce the interaction of the microfibrils. Moreover, the concentration of the swelling solutions, swelling time and the temperature were concerned in the study. Finally; the fibers were fibrillated by shearing at high speed. The fibers presented different morphological shape after sheared by different time. The degree of the fibrillation was measured by BET test, and the surface morphology of the fibers was investigated by SEM. Furthermore, the factors, such as the concentration of the etching solution, pre-treating time and temperature, may influence the degree of the fibrillation and the surface morphology of the fibers was studied as well. The results demonstrated that the specific surface area was greatly dependent not only on the concentration of the etching solution and the swelling solution, but also influenced by the swelling temperature, swelling time and shearing time as well.