Cyclodextrins (CDs) have been found to form inclusion complexes with various polymers with high specificity to give stoichiometric compounds in crystalline states. In these complexes, polymer chains were found to thread into Cyclodextrins and to be recognized by cyclodextrins. α-Cyclodextrin (α-CD) formed complexes with poly(ethylene glycol) (PEG) of molecular weight higher than 300, although β-CD did not form complexes with PEG of any molecular weight. However, β-CD formed complexes with poly(propylene glycol) (PPG) of various molecular weights, although α-CD did not form complexes with PPG of any molecular weight. γ-CD formed complexes with poly(methyl vinyl ether) (PMeVE), though α- and β-CDs did not form complexes with PMeVE. Polyrotaxanes in which many CDs are threaded on a single PEG chain were prepared by capping the chain ends with bulky groups.