The ultrastructure of the cotton fiber in Lumian No.11 was investigated with scanning tunneling microscope (STM), and the results were compared with that by scanning electron mincrocope(SEM). With SEM, fibrils could be seen, it was difficult, however, to further observe the fine structures in fibrils. Otherwise, with STM, fibrils could be seen clearly, and by making scan areas smaller, it could be observed that the fibrils were composed of microfibrils, and the microfibrils were composed of elementary fibrils, which were the smallest structural units of the fibril, which were arranged in parallel.