A　cationic　dye　pentamethyl-p-rosaniline　hydrochloride　6B　was　used　as　a　chromic　probe　and　its　binding　reaction　with　nucleic　acids　was　studied.　In　the　weak　alkalescent　medium,　the　cationic　dye　showed　a　decrease　in　absorption　at　its　maximal　absorption　peak　(584.5　nm)　on　binding　to　nucleic　acids.　Based　on　this　fact,　a　new　UV-Vis　spectrophotometric　method　for　the　determination　of　nucleic　acids　has　been　developed.　Effects　of　ion-intensity,　surfactant　and　organic　solvent　were　investigated　to　establish　optimum　reaction　conditions,　and　then　nucleic　acids　were　detected　with　quantitative　analysis.　The　calibration　curves　of　nucleic　acids　such　as　hs-DNA　sm-DNA,　ct-DNA　and　yeast-RNA　were　linear　over　the　ranges　0.50-4.00　mug(.)mL(-1),　0.20-5.00　mug(.)mL(-1),　0.20-5.00　mug(.)mL(-1)　and　0.20-4.50　mug(.)ml(-1),　respectively.　The　detection　limits　(3sigma/K)　were　0.082,　0.037,　0.038　and　0.041　mug(.)mL(-1).　In,applications　to　quantitative　analysis　of　nucleic　acids　synthesis　samples,　the　recoveries　of　DNA　or　RNA　ranged　from　93.5%　to　105.0%.　Furthermore,　the　mechanism　of　interaction　of　pentamethyl-p-rosaniline　hydrochloride　6B　and　nucleic　acids　system　was　also　discussed.