Coumarin and 7-hydroxycoumarin (7HC) were photo-dimerized in an aqueous solution of polymeric β-cyclodextrin (PβCD) to investigate the effect of the photo-dimerization on the viscosity of PβCD solution. PβCD was prepared by cross-linking β-cyclodextrin (βCD) using epichlorohydrin. The content of βCD residues in PβCD, determined by a colorimetric method using phenolphthalein as an indicator, was 28.8 wt%. The solubility of coumarin increased with increasing concentration of PβCD, because the benzene ring of coumarin was included in the hydrophobic cavity of βCD residue. PβCD also increased the solubility of 7HC, but the solubilizing effect on 7HC was much less than that on coumarin, possibly because the hydroxy group on the benzene ring was likely to suppress the inclusion in the cavity of βCD. And, the photo- dimerization degree of 7HC in an aqueous solution of PβCD was about half than that of coumarin, possibly because the vinyl ether of pyrone group of 7HC could be concealed in the cavity of βCD. The photo-dimerization of coumarin in an aqueous solution of PβCD could significantly increase the viscosity of the solution. One coumarin dimer would hydrophobically interact with two βCD residues so it can act as a cross-linker for PβCD.