It is known that double-stranded DNA (dsDNA) turns into a liquid crystalline phase by the addition of a high concentration of polymer with salt. SYBR Green I (SG) is a well-known sensitive fluorescent stain for dsDNA, and is intercalated in liquid crystalline DNA. Formation of the liquid crystalline dsDNA-SG complex has been confirmed by CD spectral measurements, fluorescence spectral measurements and confocal fluorescence microscopy. SG in dsDNA was also used as a singlet oxygen generator. We conducted photoirradiation experiments using three kinds of 42-mer oligonucleotides with SG. The amount of guanine decomposition by selective irradiation of SG was analyzed using HPLC after digestion of dsDNA in each sample solution. We found that singlet oxygen produced in liquid crystalline DNA promoted guanine damage much more efficiently than in homogeneous solution.