Inclusion complexation of various trans- and cis-cinnamic acid derivatives with α-and β-cyclodextrins in aqueous solution was studied by circular dichroism (CD), ultraviolet (UV), and nuclear magnetic resonance (NMR) spectroscopies. By the analyses of the induced CD and UV spectral changes, the spatial relationships between guest and host molecules was investigated. To elucidate the inclusion mechanism, stoichiometric ratio, which was found 1 : 1, formation constant, and thermodynamic parameters for trans-cinnamic acid-cyclodextrin systems were determined. Various factors in guest molecule such as ionization, hydrophobic nature, and substituent effect were reflected in inclusion formation. From the evidences of sign of the induced CD and NMR chemical shift, it was proposed that phenyl moiety of cinnamic acid was fixed into (R)-configuration within the cavity of cyclodextrin.