Lyocell fibers were heat-treated under different conditions. The tensile strength and initial modulus of the heat-treated Lyocell fibers increased sharply, whereas the elongation at break decreased. Moreover, applying tension to the fibers during the heat treatment further improved the tensile strength and initial modulus. In addition, the crystallinity of the heat-treated fibers increased slightly, and there was no obvious change with an increase in the tension; the general orientation of the heat-treated fibers increased, the crystalline orientation little changed, and the amorphous orientation improved. Also, the improved mechanical properties of the Lyocell fibers via the heat treatment could not be preserved for long. The reason may be that the crystalline structure of the Lyocell fibers was not destroyed and no new crystallites were formed during the heat-treatment process. Therefore, the heat-treated Lyocell fibers reverted to their original state with time because there was no crosslinking point to fix the orientation, although the cellulose molecules of the amorphous region of the Lyocell fibers were more oriented by the heat treatment with tension. (c) 2006 Wiley Periodicals, Inc.