Abstract: Scanning Electron Microscope applied to investigate the ultramicromorphological structure of pulp fibre was treated by biological bleaching. Morphological changes were found on surface and cross section of pulp fibres. The method improves osmolarity of fibre and increases bleach of fibre. The results show that crackings on surface and cross section of fibres by 1# and 2# xylanase bleaching, Its the useful method to improve osmolarity, but it does decrease the strength of the fibre; There are pores and crackles on surface and cross section of fibres by 3# xylanase bleaching. Its useful to improve osmolarity and has no effects on strength of fibre. So, 3# xylanase bleaching is a good biological bleaching.