Cotton fabrics were slack-mercerized in sodium hydroxide, ammonia water(min. 28% NH₃), and mixture of ammonia water/sodium hydroxide. The surface of treated cotton fabrics were observed through SEM, and the changes of fine structure were analyzed by X-ray diffractometer. In addition to that shrinkage was compared mutally, and tenacity-elongation compared, too. SEM photographs showed fibrls rearranged strightly toward fibre axis. X-ray diffraction represented increased amorphous fraction, and partial recrystallization into cellulose II lattic after treatment with sodium hydroxide and mixture of ammonia water/ sodium hydroxide. The mercerization produced increased fibre tenacity-elongation with slack state. Lastly, the best condition of mercerization was 4%/14%, 10℃, 5∼30 min. in mixture of ammonia/sodium hydroxide.