Poly(N-acetylethylenimine) was found to form a crystalline inclusion complex with gamma-cyclodextrin (CD). It did not form crystalline inclusion complexes with alpha-CD or beta-CD. It is a hydrophilic, nitrogen atom-containing polymer that forms a crystalline inclusion complex with CD. FT-IR spectroscopy, thermogravimetry analysis, X-ray diffraction, H-1 NMR spectra and C-13 CP/MAS NMR spectra were used to characterize the structure and property of the crystalline inclusion complex.