Liquid ammonia treated cotton fiber and fabric were mercerized with 20% sodium hydroxide (NaOH) aqueous solution for 5 min at 20°C to 100°C. X-ray diffraction patterns of the cottons were measured with a Rigaku Denki III DMAX. The NH₃-treated cotton fiber showed almost Cellulose III crystallite pattern. After mercerization, the crystallite phase transformed to Cellulose II_III and a little Cellulose I_III. On the other hand, though NH₃-treated cotton fabric was changed partially to Cellulose III_II, untreated Cellulose I crystallite was remained not a little. The obtained Cellulose III_I crystallite of the fabric was completely returned to Cellulose I_III by mercerization regardless of the temperature, Cellulose II_III crystallite was not observed despite of a decreasing of the mercerizing temperature. Therefore, it seems that mercerization of the fiber and fabric is considerably different. Apparent dyeing rate of the NH₃-treated cotton fiber and fabric with C.I. Direct Blue 1 was increased by the hot mercerizing. The rate of the fiber was increased with a decrease of the mercerizing temperature, whereas that of the fabric was no difference among the temperature.