The linear optical absorption spectra of three isomers of planar boron cluster B13 are calculated using time-dependent spin-polarized density functional approach. The geometries of these cluster are optimized at the B3LYP/6−311+G* level of theory. Even though the isomers are almost degenerate, the calculated spectra are quite different, indicating a strong structure-property relationship. Therefore, these computed spectra can be used in the photo-absorption experiments to distinguish between different isomers of a cluster.