The phase transition from cellulose to alkali cellulose (Na-Cell) and the phase structure of Na-Cell have been investigated by CP/MAS 13CNMR spectroscopy. It has been found from the 13CNMR spectra that both crystalline and noncrystalline components of cellulose decrease in resonance intensities in the transition process from cellulose to Na-Cell I, suggesting that the noncrystalline and the crystalline regions of cellulose are converted to Na-Cell simultaneously. The conversion model is considered as follows: The crystalline part imposes some restriction on the swelling of the noncrystalline part of cellulose, but the conversion of both parts to Na-Cell may be possible when the swelling prevails in the whole microfibril as a result of the penetration of the alkaline solution into the crystalline part. Secondly, the 13C spin-lattice relaxation times (T1) have been measured to elucidate the phase structure of Na-Cell. Though two T1 values are obtained for each carbon of Na-Cell except for C6 carbon, Na-Cell has a homogeneous solid structure which is composed of the crystalline regions with a small heterogeneity.