We investigated a series of carbocyanine dye dimers formed in a cyclodextrin (CD) nanocavity by analyzing their steady-state absorption spectra in aqueous solution. As increasing CD concentrations, a blue-shifted additional peak was enhanced in several dye/CD combinations. This blue-shifted peak indicates that a 'sandwich-type' dimer is formed in CD. In order to obtain dimer-formation constants, we fitted dimer/monomer absorbance ratio to a theoretical equilibrium equation and the dimer structure was estimated on the basis of observed energy splittings in the absorption spectra. These values are significantly influenced by steric factors (poly-methine chain length, bulkiness of the end-group, CD cavity-size, etc.).