Simultaneous measurements of WAXD, SAXS, and Raman spectra have been performed during the water-induced phase transitions of poly(ethylenimine) (PEI). As is well-known, PEI transforms among four kinds of crystalline forms by absorbing water: anhydrate (EI/water = 1/0), hemihydrate (1/0.5), sesquihydrate (1/1.5), and dihydrate (1/2). The chain conformation changes front double helix in the anhydrate (0) to planar-zigzag form in the hemi- (0.5), sesqui- (1.5), and dihydrates (2). The structuctural changes in the crystalline region during the water-induced phase transitions have been found to induce also the remarkable changes in the higher-order structure of stacked lamellac oil the basis of the detailed comparison between WAXD and SAXS data. The anhydrate (0) composed of double helices showed the long period of 81 angstrom. When the anhydrate (0) started to transfer to the hemihydrate (0.5), the two types of long periods, 75 and 159 angstrom, were observed. The 159 A period appeared only temporarily in the transition process from anhydrate (0) to hemihydrate (0.5). In the transition from hemihydrate (0.5) to sesquihydrate (1.5) and to dihydrate (2) the long period changed to 64 angstrom (0.5 and 1.5)and 61 angstrom (2). These SAXS profile changes have been reproduced quantitatively on the basis of it stacked lamellar structural model with paracrystalline disorder taken into consideration.