Mercerization is a typical transfer reaction from cellulose I (Cell I) to cellulose II (Cell II) and it is recognized that this transfer is irreversible. To clear the mechanism of the transition from Cell I to Cell II, the change of the crystal structure and the chain conformation during the mercerization are studied under various conditions by means of wide angle X-ray diffractograms and CP/MAS ¹³C NMR spectra. It is observed that Cell I and Cell II show the different ¹³C NMR spectra resulted from the different chain conformation, as proposed by Hayashi et al . In sodium celluloses treated with and without fixing the fibers, which show the similar X-ray diffractograms, CP/MAS ¹³C NMR spectra show different patterns which result from two types of the chain conformation, that is Na-Cell of Cell I type and of Cell II type. By regeneration, the Na-Cell of Cell II type transforms to Cell II independently on the temperatures of washing water. On the other hand, the Na-Cell of Cell I type mostly transforms to Cell I by washing with water at about 100 °C.