Drying-induced hornification is an inevitable phenomenon of cellulosic fibers, which is used to describe internal aggregation structure changes of cellulosic fibers upon drying or water removal. To investigate the hornification process, never-dried cellulosic fibers with different components were thermally dried to different moisture contents. The results indicated that the hornification process could be divided into four stages, including the first crystallization period (>70% moisture), the cocrystallization period (70-31% moisture), the hemicellulose control period (31-11% moisture), and the second crystallization period (11-0% moisture). The decrease of water retention value (WRV) occurred in the cocrystallization period and the second crystallization period, which meant hornification happened in these two periods. Besides, hemicellulose and lignin inhibited hornification by reducing cellulose cocrystallization. The work elucidates the hornification process and mechanism of cellulosic fibers,which will be helpful to control the properties of cellulosic materials for extended utilization.