A supramolecular inclusion complex between β-cyclodextrin(β-CD) and α-naphthylacetic acid was prepared, and its crystal structure was investigated by single-crystal X-ray crystallography. The complex contains two β-CD molecules, one α-naphthylacetic acid, two ethanols and twenty-eight water molecules in the asymmetric unit, which could be formulated as [(C42H70O35)2·(C12H10O2)·(C2H5OH)2·28H2O]. Two β-CD molecules constitute a dimer by face-to-face contact of their secondary hydroxyl sides. At the interface of the dimer, one α-naphthylacetic acid molecule is sandwiched between two β-CD molecules. Each β-CD unit of the dimer includes one ethanol molecule in its cavity. The β-CD dimers are linked together via hydrogen bonding to form layers that are stacked in a brickwork-like pattern. The comparative study of some sandwich complexes elucidates that the interface of the β-CD dimer has a stronger inclusion capacity than the cavity of β-CD for some suitable planar guest molecules. The novel inclusion structure results from the competitive inclusion of α-naphthylacetic acid and ethanol.